It was not because you were more in number than any other people that the Lord set His love upon you and chose you.

Deuteronomy 7:7

Dr. Martin Luther continues with his running commentary on this great book Deuteronomy. He writes concerning Moses:

See what a careful expositor of the First Commandment Moses is.

Above he had taught the true worship of God in faith, fear, and love of the Spirit; here he has come to the outward work.

But first he issues a salutary warning, lest they complete in the flesh what they had begun in the Spirit, and be puffed up by their own work. Then, when the godless and the idols were outwardly destroyed, they would themselves become more godless and would set up worse idols in the heart, namely, trust and glory in the deed (as the flesh is wont); and thus a beautiful work would cause them to sin more gravely against the First Commandment than the very Gentiles whom they had slain.

So he keeps them at the rightful use of the Law by wholly removing boasting and trust in works, and he calls them to trust in grace alone, saying: “Not because all the nations, etc.” as though he were saying: “The fact that God uses your sword does not happen because He needs your power or because He cannot do it without you, since you are very few in number. The glory of the work is not yours, but His, who by using your small number destroys such a great multitude. Otherwise, if He had wanted to conquer with a multitude, He would not have chosen you but other peoples who are much more numerous than you.”

What, then, is left in this work about which you can boast? Nothing of yours; but “because the Lord [he says] chose you and kept His oath, etc.” (v. 8). You see that nothing is held out to human trust in any work but the undeserved love of God, by which He is moved to approach us with His Word and promise even before we are born. It is out of the question that He should requite anything after we are born or begin to serve Him.

And this is the pure and unalloyed meaning of the First Commandment: We should deem ourselves to be nothing as regards our merit, but to have, receive, and find power to do everything only by His mercy and love, to His glory—mercy which He first promises by His Word and then also confirms afterward by a work which He does through us, as by a sign, just as here He cites the Exodus from Egypt and the destruction of the Canaanites.

LW 9 Pg.84-85

He continues in verse 17 and 22: If you say in your heart: These nations are greater, etc.

Moses takes away another occasion for transgressing the First Commandment, namely, mistrust, which arises from the weakness and infirmity of our power, if it is compared with the work and the Commandment. That is how those spies sinned who frightened the people away from entering the land, which they claimed contained giants, called the Anakim, and walled cities, as we saw above.

But here Moses teaches again how the Commandments of God are to be kept, namely, not by our powers but by the divine strength that is promised to us. With one word he quenches at the same time both the trust and the presumption of our power and the mistrust and the despair of our infirmity—presumption by this, that he shows that greater things are commanded than we can do, so that here he admits that the Gentiles whom He commands to destroy are more numerous and powerful than Israel, their destroyer; despair by this, that through them the Lord will do everything He commands them.

If, therefore, the Children of Israel had looked at their strength and compared it with the strength of the Gentiles who were to be destroyed, they would have despaired completely and paid no attention to the commandment of God. Now, however, that they may carry out everything by faith, He will promise that God will stand by.

To the promise He adds the example of His previous mercy, by which He freed them from the hands of Pharaoh and the Egyptians, in order to hearten them by word and deed to the faith by which they would destroy the Gentiles at the command of God. Hence He also declares with a new promise that He will send hornets, that you may see how much is involved in the strengthening of faith.

For through faith God is served; through faith the commands of God are fulfilled; through faith we deserve having the divine power stay by us in all our works, as Christ rightly said (Mark 9:23): “To him who believes all things are possible.”

LW 9 Pg.86-87

And he comes to a wonderful conclussion in verse 22:

How persuasively and paternally He invites them to faith!

He anticipates even their future weak thoughts of faith, so that when the promises of God begin to be fulfilled—because the Gentiles have, in large part, been destroyed, and others are left—they should not constantly imagine that they are forsaken or deceived by the promise.

Everything is happening for their good, to enable them to take over the land more firmly and fully and to prevent them from being forced to bear even crueler beasts in place of the godless people who were destroyed. But this very postponement is given also for the converting of the Gentiles, that those who wish may come to their senses.

LW 9 Pg.88