Look at Moses. He makes the point that nothing must be added to or subtracted from the words of God, lest the Commandments of God be not kept. By saying this he shows that in a sacred matter the traditions of men conflict with the words of God, since they resist and cause offense, so that the Commandments of God are not kept. With this name Paul, too, brands them with infamy (Titus 1:14), teaching that they turn aside the truth, and (Rom. 16:18) that they lead the hearts of the innocent astray. And Christ charges us (Matt. 16:6) to beware of the leaven of the Pharisees and of wolves in sheep’s clothing (Matt. 7:15).
Luther’s works, vol. 9: Lectures on Deuteronomy 1525