Several lectionaries appointed selected verses of Deuteronomy 4 as the First Lesson for last Sunday. The Christian Worship lectionary appoints Deuteronomy 4:1-2,6-8 as the First Lesson for this coming Sunday; the Christian Worship: Supplement lectionary adds verse 9 to the pericope. So this quote may be a little late for many others, but for WELS pastors planning for September 6, 2015, this applies to the upcoming Sunday. I appreciated this quote from Luther’s comments on Deuteronomy 4:2 regarding the command to neither add to or subtract from the Word of God, which then lead into Luther’s comments on free will.
Verse 2. You shall not add to the word which I command you, nor take from it.
Before Moses establishes or teaches anything, he first of all sees to it that it be kept unaltered and untouched, lest anything be added or removed. In one short expression this passage condemns all human laws, and it decrees that in a matter of conscience nothing but the Law and Word of God is valid. And in this the will of God, which thus judges, ought to be enough, even though both reason itself and necessity demand the same thing. For since through the sin of Adam we are sunk in blindness, so that we are wholly ignorant of God in all His will and counsel, it is not only foolish but also impossible of ourselves to prepare a light and a way by which to approach God and find out what He would have us do, as He says in the Book of Wisdom (9:13–14): “The thoughts of mortals are fearful and uncertain. For who among men can know what God wants?”
Unless, therefore, God reveals His Law, by which He makes His will known to us (Ps. 103:7), there remains only that saying of ours: “Every man is a liar; every man, vanity” and Is. 55:9: “Just as the heavens are above the earth, so are My ways above your ways.” Hence that argument of the scholastics about free will is most foolish: “I have the freedom to govern a cow or to throw away money; therefore I have the freedom to do what pleases God and to serve Him.” But that is the same as if you said: “I can tread on the earth with my feet and walk on the earth; therefore I can also go into the heaven and walk on the clouds.” Indeed, from his creation man has free knowledge and power to rule and deal with those lesser than himself. But to govern himself and to do what pleases his superior, he neither knows nor is capable of. There free will ends; there he is necessarily blind, powerless, yes, dead and condemned. Therefore one should not be presumptuous here or choose what to do; but one should lean only and solely on the Word of God, neither adding nor subtracting anything. Hence where a man is not altogether certain that the Word of God is present, there he should not go at all; where he is certain that the Word of God is present, he should not draw back. This is such necessary counsel.
Luther, M. (1999). Luther’s Works, vol. 9: Lectures on Deuteronomy. (J. J. Pelikan, H. C. Oswald, & H. T. Lehmann, Eds.) (Vol. 9, pp. 50–51). Saint Louis: Concordia Publishing House.