This passage condemns… representations of God, which He wanted to be forbidden to a simple and childish people, but … inner representations of God, which are (as we have said above) opinions and speculations about God constructed out of ourselves without the voice of God. Here indeed the voice of His words alone is commended, and whatever is said or thought about God which is not that voice of His words is wholly godless and damnable. He wanted to have His will and His counsels delineated for us by His words alone, not by our thoughts and imagination.

Therefore it is not what seems to you to please or displease God, no matter how holy or pious it appears to you (as the founders and confessors of religions and sects have supposed), that pleases or displeases God, but what He Himself by the voice of His Word designates as pleasing or displeasing to Him.

For nobody but God Himself describes or indicates the will of God; therefore everyone errs in a godless manner if he tries this, since nobody knows the depths of God except the Spirit who is in God (1 Cor. 2:11). Therefore it is impossible for men to think properly about God, speak about Him, or worship Him, without the Word of God. The affirmation stands: “You have heard His voice.”

Martin Luther in his private lectures on Deuteronomy (LW 9, Pg. 58)