Jesus Christ says: Abide in Me, and I in you. As the branch cannot bear fruit by itself, unless it abides in the vine, neither can you, unless you abide in Me.

Gospel of St. John 15:4

Luther comments this (Listen to the German audio!) and writes – according to the translation into English:

“You now have My Word,” Christ says, “whereby you are clean; through it also your fruit is good, and everything pleases God. But if you want to retain this cleanness and to keep on bearing good fruit, see that you remain in Me through faith and do not prize your own deeds presumptuously or falsely rely on them. Nor dare you let any trials alienate and tear you from the faith. Otherwise you will both forfeit the cleanness which you have in Me and utterly spoil the fruit.”
These words are spoken for our admonition and warning. If it were not for our headstrong ways, they would be terrifying indeed. For here is Christ’s blunt judgment: “Whoever does not remain in Me can bear no fruit and will be cast away like a withered and useless branch. Hence if you are to bear fruit and if your words and your doctrine are true and your life good, then be concerned that you remain in Me and that in no circumstances you be found outside of Me.” But as it happens, most people live under the illusion that it is unnecessary to be and remain in Christ, since it is evident that other people—the Turks, for instance, and false Christians—bear many fine fruits. In fact, many of their works surpass those of true Christians; they lead fine, respectable, rigorous, and ascetic lives, establish and conduct elaborate services of worship, and give and suffer much for the sake of God. And many comment on this verse sneeringly and say that, after all, much can be accomplished without Christ; for we see people build; rule over land and subjects; maintain justice, peace, and order; and do much additional good—and all this with the powers of reason and nature. In view of this, the meaning of these verses—“You cannot bear fruit unless you abide in Me” and “Apart from Me you can do nothing”—must be that Christ is Lord over nature; for He and nature do not create anything to no purpose. But let those mockers have their way until they have mocked their fill!
According to His own interpretation, Christ is speaking here of such fruits as remain eternally, have no end, and are praised and honored before and in the presence of God forever. And these are not natural fruits; for concerning natural fruits we know well enough that among the heathen and the unbelievers the begetting and rearing of children, the governing of lands and people, and the like, may be just as good as, and even better than, they are among the Christians. We know that the heathen also have the same physical life and existence, just as they, like us, have all kinds of gifts. Therefore no one need teach us this or interpret Scripture to mean such works as God’s Word does not teach but leaves as they are and as reason has been ordered to control them.
Scripture, however, instructs us about such fruit as serves for eternal life. The other works do not interest and please God beyond the point where they pertain to and touch on this physical life. None of them will endure up there, but the world and all its ways must perish here and be buried. Therefore this text must be taken to refer solely to a life conducted so as to please God both here and there—a life which will never be forgotten. This is “enduring fruit”—so Christ calls it—which we will take with us and of which God will testify on the Last Day: “This is fruit that is pleasing to Me. I will reward it eternally.” No heathen or Turk will ever hear these words, even though he is found to be a pious and respectable man or woman, prince, burgher, or peasant. For their fruit is natural and good, to be sure; but it is not Christian and everlasting.
Thus Christ says here: “If you want to bear true fruit—fruit that is acknowledged by God—you must remain in Me; for I am speaking of a class, activity, and fruit higher than what Moses writes about in Gen. 1:28, where we read that after God had created heaven and earth and all it contained, He turned it over to man and commanded him to rule. I am speaking of the works that are needed when the reign of Moses and nature terminates, when this life and all its activity ends, so that one may know where to stay when death comes and takes everything away. Neither the heathen nor the world can be familiar with this. But you alone, as Christians, shall and can know it and bear fruit of this kind, provided that you remain in Me.”
With these words Christ again looks at all Christendom and sees what the state of affairs always is and will be when this message is proclaimed: “He who believes in Me and is baptized shall be saved (Mark 16:16). He who wants to be redeemed from sin, death, and the devil, let him call upon Me. That is the message that shall and will be preached of Me.” But where this is done, there, first of all, the devil and the world will want to smother it violently. Failing in this, the devil will enlist factions and sects. There will be many who initially accept the message with joy and become Christians; but when matters become serious and they have to suffer for this, or are in danger of losing life, goods, and honor, they put their whistles in their pockets and reason: “I will remain a Christian and at the same time keep what is mine.” Thus they mock Christ, as though they were unwilling to deny Him but still wanted to conduct themselves in such a way that it would not be necessary for them to be hated and persecuted by the world.
Furthermore, there will be many who will speak proudly of Christ and say (Matt. 24:23): “Lo, here is Christ,” or “There He is!” The popes and the mobs of monks belong in this category. They say: “He who follows us, observes such and such rules, lives in such and such a manner, is a true Christian.” They will create many sects, so that many people will be offended and, as Christ Himself declares in Matt. 24:24, “even the elect might be led astray. As a consequence, only a few will preserve My Word in its purity and remain in Me.” For they all champion a doctrine that is acceptable to our reason and easily conforms to it. Reason will always hold to and teach what it considers right, but it will reject what bewilders it. Thus the Anabaptist rabble drivels that Baptism cannot cleanse man from sin; for, as they say, it is only plain water, and an external substance cannot help anyone obtain remission of sins; the Spirit alone must do this. Therefore, they say, Baptism is no more than a seal and token for Christians; it certifies their membership in Christendom. The Arians blasphemed: “Do you suppose that Christ, the Virgin’s Son, could be true God, coequal with the Father?” And whatever similar heresies there are, they all are doctrines which reason can fathom and is glad to believe. And they all claim to be in Christ and glory in Him. They say: “Lo, here is Christ! This doctrine is true.”
The faithful Savior Christ foresaw how difficult it would be to remain in Him in the face of the many obstacles that the devil would throw in the way—here with violence and murder, there with lies—for he is a murderer and a liar from the beginning (John 8:44). Therefore wherever truth dawns, he strikes the branches of the vineyard with so much hail and lighting that He severs them from Christ, the Vine. He uses fire, the sword and whatever means he has at his command. He would like to chop down and uproot the stock. And then there is the specious excuse with which the devil comforts people and causes them to say when they are menaced with persecution for the sake of the Gospel: “What shall I do? After all, I must obey the government. I dare not forsake wife and child.” Or if the devil cannot subdue the Gospel with violence, he sends his vile ravens, sparrows, and other vermin and noxious creatures, that is, false doctrine and sects, to eat and ruin the vine.
“Therefore,” says Christ, “mark this well and heed My warning; it will require great exertion and be very dangerous to remain in Me. For he who will do so must endure the murderer and liar, and must be strong enough to resist him and ward him off, to keep from being torn away either by violence or by cunning. I tell you this in advance, that you may know that it will not be easy and (as He says later) will not be achieved without much fruit. For I tell you truly: As the branch cannot bear fruit by itself unless it abides in the vine, neither can you, unless you abide in Me (John 15:4). There you have My concise and clear opinion. Therefore do not be deceived by the false, beautiful, and gleaming fruit which is seen on those who persecute the Gospel and claim that they alone are the true Christendom. They do many great and excellent works and put forth far more effort than the true Christians. (The monks do this today and have always done so, just as I also did when I was a monk.) But be on your guard against such works! For they may be called fruit, but the monks are not and do not remain in Me. This stamps their works as nothing but false, accursed lies and rotten fruit in the sight of God, even though outwardly they appear precious before the world. Evidently the monks imagine that they have the true fruit and are God’s bosom friends. They regard you as heretics, apostates, and rejects. And if you want to be guided and to judge by such appearances, you will soon become confused and will not know who are true Christians. Therefore see that you remain true in Me and that you do not permit the hypocrites, who claim to be the true branches, to mislead you, lest you, like them and with them, be uprooted and cast away.”
You say: “How can I know which is the true fruit? Or how can I identify those who remain in Christ?” Answer: Christ has just declared: “You are already made clean by the Word which I have spoken to you” (John 15:3), and one must not look to see how great and how numerous their works are. For all these works are no more than what even non-Christians and knaves are able to do. No, one must look above all at the lips to learn what they teach and believe, whether it is the pure Word of Christ or not. One must ask first of all: “Do your deeds, your suffering, your worship of God proceed from the Word which Christ proclaims?” Then we will soon say whether it is true, pure, and good. For if this is not the case, we can judge without hesitancy that it is not pure and that these are not good fruits from this vine but are already condemned by Christ, even though you were to torture yourself to death or raised the dead every day. For it does not abide in Christ, since it does not have His Word.
Now one sees that the chief doctrine of the pope and of all factions is this: If you want to be rid of sin and to be saved, you must not only believe; but you must also live in such and such a way and do and suffer a specified amount. Yes, in the past our papists understood and taught nothing at all about faith; now they have learned this little word “faith” from us. They are ashamed of their former doctrine and are beginning to touch it up. Now they do speak of faith; but they append the doctrine of meritorious works, thereby nullifying faith. And the worst of it is that after teaching it so long they have finally sealed this with their stench and have declared publicly: “A man may be ever so pious and perform ever so many good works, yet he does not know whether he attains mercy before God.” That is what they say about the pious people, who are upright and perform good works but are not to know how they stand with God. The vile and accursed devil from hell told them to say and proclaim this!
From this we can realize what their fruits are and how little their doctrine conforms to the true Word of Christ. For in the first place they say nothing about faith. They do not say that we have forgiveness of sin through Christ, but they command me to do good works in order that I may be cleansed from sin. And yet, after I have performed all these good works, I am neither to know nor to believe that I have a gracious God, although Holy Writ teaches me that I must believe and know that God is gracious to me through Christ before I can do works that please God. On the basis of this I can conclude freely that he who teaches, believes, and lives thus is not in the vine but is a doomed branch, condemned with all his fruit and deeds, since the only thing they teach is that one must always doubt and can never be certain of being in Christ—that is, in God’s grace—and of bearing true fruit. For this reason alone the papacy must be condemned as the seduction of the devil, even if nothing else in it were reprehensible. How else could I account for this misery that I am to regard myself as a Christian and to practice the good works which they teach, torture myself to death over them, as some of them have done, and still be in doubt and say: “God knows whether I am pleasing Him and have His mercy!” Let the devil do this; I will not. Should a prince, a burgomaster, or a father work hard, worriedly, and with discomfort to perform the duties of his station and office and never know whether the best he has accomplished is pleasing to God? In the end, who would want to serve God any longer or do and suffer anything for His sake? Yet under the papacy such accursed, unchristian teaching has made its erring way throughout the world and has been practiced in all the schools and from all the pulpits.
Scripture and the doctrine of the Gospel, however, teach us that it is essential for us to be convinced first of all that God is our gracious Father—which happens if we believe in Christ—and then construct all our words, deeds, and life on this foundation. I must be able to say: “I know that I have a gracious God and that my works, performed in this faith and according to His Word, are good fruits and are pleasing to Him.” Wherever there is the kind of preaching that assures hearts of how they stand with God, I can conclude that such a sermon is true and presents the pure Word of Christ. On the other hand, I can judge that any other message is a lie and the devil’s doctrine, which turns the two things around and declares: It is not for us to know whether we live in grace, but we must promote and perform good works at random and with doubts in our minds. One may reasonably say to them: “If I am to hear no other comfort from you than this, that I can never know how I stand with God, then be the devil’s confessor, and be a preacher in the abyss of hell!”
Now you ask: “But how can I be sure that God is gracious to me and that my works are pleasing to Him, since I am full of sin and unworthy?” My answer is: “Why, then, do you perform such works and teach that sins are atoned for by them if you do not believe that those works are pleasing to God? It would be just as well for you to remain as you were before as it would be to perform these works in unbelief or in delusion and thereby incite God’s wrath all the more. For since you have that view, God cannot be gracious to you or be pleased with your deeds. No, you will receive just what you expect of Him and in accord with your doubts regarding His mercy, and you will never be able to rid yourself of such thoughts. Therefore we say: If you want to deal with God and not come to grief, you must above all else be able to say with conviction: ‘I know that this work that I am doing is a good work and that it is pleasing to God.’ ”
But from what does such a conviction stem, or how can one conclude with certainty that God is gracious to us and pleased with us? To be sure, not by and of yourself, but solely because of Christ’s words “Unless you abide in Me”; for “as the branch cannot bear fruit of itself, unless it abides in the vine, neither can you, unless you abide in Me” (John 15:4). Therefore we must not look at ourselves to see what we are and what we do, whether we are worthy and our works are sufficient. Otherwise the doctrine of the papacy and the statement of monks and priests would be correct: “Who knows whether we deserve mercy or displeasure?” For they do not look beyond themselves, their vocation, or their works but presume to reconcile God and obtain mercy through these, just as the heathen and the Turks do. They do not know what it means to be in Christ. Therefore it is impossible for them ever to conclude with certainty that their calling and life are acceptable to God. They are doomed to eternal doubt; and the more they work and torment themselves in an effort to please God, the greater their uncertainty becomes, until finally it culminates in despair. This is the inevitable lot of all who rely on themselves and their deeds. For in the final analysis we will find that so far as we are concerned all our works, even the best, are sinful and damned to hell in the sight of God. God’s Word has already pronounced wrath on man’s own ability, power, and works. Whatever does not abide in this Vine Christ must be condemned and exterminated.
Therefore he who wants to be helped out of such doubt should be intent solely on coming out of himself and all his works into Christ and on learning to know how we come to grace through Him, are pleasing to God, and thus through faith are grafted into Him as branches. Then he can say: “I know, praise God, that unfortunately I am a poor and unworthy man and have deserved nothing but hell and wrath before God; but I also know that God is gracious to me for the sake of Christ the Lord, who suffered and died for my sin. And since I am in Christ and am cleansed by Him, God takes pleasure in my life and works, which proceed from such faith, and regards them as good fruit.”
Thus I can speak differently about my vocation and my activities from the way a heathen, a Turk, or an unbelieving saint can speak; for I am not only a prince or the head of a household, a man or a woman, who administers an office or vocation as the others also do; but I am also baptized and washed with the blood of Christ. This has nothing to do with my station or calling in life. For Baptism does not make me a prince, a subject, a husband, or anyone else; but it does make me a Christian. Furthermore, I also have the Word, which tells me that Christ died and rose again for me. This same Word makes no one a priest, a monk, a master, a servant, etc.; but it does create a heart that receives God’s grace and is cleansed by faith. This is what it means to be and remain in Christ. Then they may preach to me what they please; I adhere to the fact that I am baptized, not to my life and my vocation but to the Man called Jesus Christ. Through Him I am in grace and have forgiveness of sins. Similarly, when I hear the Gospel, I hear nothing about myself or about my works that could justify me before God; I hear about Christ, who has been given to me by the Father for my redemption from sins and eternal wrath. Thus through the Word and Baptism you have a reliable testimony and a confirmation. You need no longer doubt and waver, but you can and should have the conviction that you have a gracious God and Father in Christ.
Wherever there is such faith and assurance of grace in Christ, you can also confidently conclude with regard to your vocation and works that these are pleasing to God and are true and good Christian fruits. Furthermore, such temporal and physical works as governing a land and people, managing a house, rearing and teaching children, serving, toiling, etc., also develop into fruit that endures unto life everlasting. Thus the holy patriarch Abraham and our holy ancestress Sarah will be commended and praised on Judgment Day for their marital life. Although the married estate will come to an end and be no more, as will all the life and activity of this world, yet this holy Sarah, and others with her, will receive their little crowns because they were pious spouses and mothers, not by reason of their works per se—for these had to cease—but because they did these works in faith. In like manner, the works of all Christians are performed to God’s everlasting pleasure; they will not be despised, as will those of non-Christians, but will have their eternal reward also in yonder life, because they are works done in Christ and grow from the Vine.
From this you can see that it is intolerable to declare in Christendom that we cannot and must not know whether God is gracious to us. He who wants to be a Christian pastor or a believing Christian must teach and profess the opposite and say: “I know that I have a gracious God and that my life is pleasing to Him.” After all, I know whether I believe in Christ, that is, whether I adhere to His Word, which is preached to me, whether I remain true to my Baptism or fall away from it, etc. And if I do believe, then it must be certain and true that God is my gracious Father. Over and above this, I have the pledge and seal of Baptism and the Blessed Sacrament, given to me by Christ.
And if I thus remain in Christ, then it is certain that for His sake my vocation, my life, and my works are also acceptable to God and are precious fruits in His sight. And though I myself am still weak in the faith, and though many frailties and sinful lusts still dwell within me and always manifest themselves, this will not be reckoned against me but will be forgiven, provided I do not yield to them, give them free rein, or let myself be torn from faith and from the Vine. For as long as the branch is rooted in the stem or the stock and retains its sap and strength, its fruit must also be and remain good, although here and there it may be punctured by worms or infested with caterpillars and other vermin. Likewise, as long as man remains in Christ and receives and retains sap and strength from Him through faith—Christ works in him with His power and the gifts of the Holy Spirit—the weakness still inherent in him and incited by the devil and his evil nature cannot harm him. But man must constantly resist and combat this with the weapons of faith. He must remove such vermin. But if you surrender or pervert the doctrine of faith, as the papists and other sects do, and transfer your trust from Christ to your own holiness or live in open sin and shame and yet boast of the Gospel and the Christian name, you are to know that you are a false branch and no part of the Vine but are condemned and rejected together with the wood and the fruit and belong in the eternal fire.
Therefore Christ warns us all to beware and to be most carefully on guard. He means to say: “Much offense will appear among you, and many sects will creep in. You will have the devil in front of you and behind you. He will attempt to tear you away from Me. But just hold firmly to Me, that your faith may remain pure and firm, and that your life and deeds may be in the path of that faith. Then you will have no doubt or worry about how you stand with God. You need fear no wrath. Pay no attention to the accursed doctrine that says: Even if a monk has tortured himself to death with works, fasting, vigils, and self-castigation, he still cannot know whether he has merited heaven or hell. For what else is this than declaring: ‘I do not know whether Christ is speaking the truth when He says that he who believes in Him and is baptized in His name shall be saved’ (Mark 16:16). It also denies and nullifies the Creed prayed by the children: ‘I believe in Jesus Christ, our Lord, who suffered and died for us.’ Or: ‘I believe in the forgiveness of sins.’ And it is tantamount to saying: ‘I do not know whether it is true that Christ feeds me in the Sacrament with His body and blood for the forgiveness of sins.’ ” What more terrible blasphemy could be devised against Christ and His holy Word? Therefore anyone who retains such thoughts in the hour of death and passes away will surely go to the devil and remain in eternal disfavor and damnation, as one who calls Christ a liar and denies His suffering and death, Baptism, and the Sacrament.
But he who wants to be saved and go to heaven when he dies must think and say: “Have mercy on me, gracious God. I am a poor, sinful being who has merited nothing but wrath. But whether my life was good or evil, I know that I need not doubt that I was baptized and named a Christian for the remission of sins; that Christ my Lord was born, suffered, died, and was raised from the dead for me; that He gave me His holy body and blood as food for the strengthening of my faith; and that I have been absolved and relieved of my sin in the name and by the power of Christ.” Such a heart and such a faith cannot fare adversely or be lost any more than God’s Word can fail or be false. This I can guarantee you, since God Himself guarantees it to you through His Word.
But this doctrine fares just as it always has fared. Christ points out here that not all will remain in Him, with regard both to doctrine and to faith. There has never been a faction or a sect which, like the pope, did not also teach contrary to this doctrine and lead people away from Christ to uncertain trumpery, so that they no longer remain in Christ. At best the sum and substance of their doctrine is this: Faith alone does not do it; we must put forth our own best efforts; we must forsake everything. Thus in the end they concentrate on works and always remain in doubt. They do not reach the point where they lay the proper foundation stone and teach: “Before I begin to do a work, I must first be assured of the grace of God in Christ.” After this foundation stone has been laid, you must go on to perform as many good works as possible, and you must thank God, who accepted you in His mercy before and without all your works. They do not want to see or hear this little story, but they berate and condemn us because of it.
But when we call on them for something better, they prattle away insolently with their old trumpery: “You must, of course, be pious, perform many works, and suffer much.” But if you question them further: “For what purpose? What good does this do you?” they say: “Well, if it is God’s will to recognize this, He is gracious to you.” They are pulling your leg; for no matter how long you follow their doctrine, you are just as uncertain as you were before. But this is what Christ teaches: “If you want to be sure, you must, above all, be in Me before you can perform a single work or bear fruit. All your works must proceed out of and from Me. After this, however, you, too, can bear fruits that really are good. But if you reverse the order and want to bear fruit before you are in Me, neither the branches nor the fruit will endure.” For who ever heard of a branch growing from a grape? Must not everybody say that the vine and the branches must precede the fruit? For the grapes do not make the vine; the vine produces and bears the grapes. Thus we must be in Christ before we are able to bear fruit and perform good works.
This fact is so clear and certain that everyone must agree, even our papists themselves, if they could honor the truth and see and hear us and our doctrine without the prejudice of their hateful eyes and poisoned ears. They resemble the Pharisees and the scribes, who were bitterly and murderously hostile to Christ. In consequence, everything Christ said and did was sheer poison in the ears and in the eyes of the Pharisees and the scribes. This impelled Christ to say to them: “How could you do or speak that which is good, since by nature you are evil worms?” Similarly, we say: How can such people comprehend this doctrine and bear good fruit, since they are not in the Vine, yes, since they refuse to hear and tolerate His Word? They themselves cannot deny that tree and stalk must precede the fruit. Yet they refuse to agree to this when we teach it on the basis of the Word of Christ. They call it heresy and claim that we forbid good works, which is the same as if the vine were to say to the gardener: “You are ruining the stem, for you are growing branches before you make wine. First produce the grapes, and then fine branches will develop.” The vinedresser would surely retort: “You dear fool, it is obvious that you have not yet seen many branches or grapes.” Our adversaries are just as stupid and foolish. They cannot understand that no one can perform a good work, one that is pleasing to God and can be called good fruit, before he becomes a believing Christian as a branch on the Vine. Now this doctrine of ours, because of which they persecute us, is so clear and manifest that they stand convicted before God and the world. We can call upon all winegrowers, peasants, and gardeners to testify that the stem, the tree, and the stock must grow before one can produce a pear, an apple, a grape, or any other fruit. That is part of nature, yes, it is inscribed in every creature. Therefore they must all bear witness to this doctrine against the shameful perversion and lies of the devil.

Luther Works Volume 24

1. Thy body, given for me, O Savior,  Thy blood which Thou for me didst shed, These are my life and strength forever, By them my hungry soul is fed. Lord, may Thy body and Thy blood Be for my soul the highest good!

2. With Thee, Lord, I am now united; I live in Thee and Thou in me. No sorrow fills my soul, delighted It finds its only joy in Thee. Lord, may Thy body and Thy blood Be for my soul the highest good!

3. Who can condemn me now? For surely The Lord is nigh, who justifies. No hell I fear, and thus securely, With Jesus I to heaven rise. Lord, may Thy body and Thy blood Be for my soul the highest good!

4. Though death may threaten with disaster, It cannot rob me of my cheer; For He who is of death the Master With aid and comfort e’er is near. Lord, may Thy body and Thy blood Be for my soul the highest good!

5. My heart has now become Thy dwelling, O blessed Holy Trinity. With angels I, Thy praises telling, Shall live in joy eternally. Lord, may Thy body and Thy blood Be for my soul the highest good!

Friedrich Christian Heyder (1677-1754)