11. God knows the thoughts of men, that they are vain. This verse explains nearly all that has gone before. For it says what those things are which the sinners utter and speak and by which they humiliate and harass the people of God, namely, that they are vain thoughts. But that “thoughts” are here put for the wisdom and teachings of men is proved by 1 Cor. 3:20, where the apostle, in order to distinguish between the wisdom of God and of the world, cites this verse thus: “God knows the thoughts of the wise, that they are vain.” For “they spoke vain things each to his neighbor” (Ps. 12:2), because they utter and speak iniquity, that is, by preferring vanity to the spirit and the truth.
Luther’s works, vol. 11: First Lectures 2