1. Blessed are those whose way is blameless, who walk in the law of the Lord. I have not yet seen this psalm expounded by anyone in a prophetic sense, nor is there anyone who has kept the sequence and order of exposition in it without doing violence to and twisting the verses and words. I think this comes about because they did not seek first the prophetic, that is, the literal sense, which is the foundation of the rest, the master and light, the author and fountain and origin; but perhaps more so because the sense is hidden and speaks very much in the inward spirit, whence its cause and motivation, why the psalmist has spoken and arranged it in such a way and not otherwise, cannot be seen by those who do not have the same motives. For no one understands another in spiritual writings unless he savors and possesses the same spirit. But we, having now called upon the arm of the Lord, will examine the prophecy. This will be easier than to hunt for the moral dispositions one by one. They are in any case “the ways of a man in youth,” of which we are utterly ignorant (Prov. 30:18–19). Youth, I say, which is the soul itself, devoted in spirit, or spiritual through Scripture. In both it is the incomprehensible way of Christ, that is, holy inclinations, how many, when, in what manner, and to what extent they might be on the way. Therefore I believe that the Holy Spirit has reserved the full moral understanding of this psalm to Himself.
Therefore the prophet looks with spiritual eyes at the law of Moses and sees hidden and enclosed in it the law of faith, the Gospel of grace, and the invisible things promised, like the kernel under the shell or the treasure under the ground, as the Lord says: “The kingdom of heaven is like treasure hidden in a field” (Matt. 13:44), that is, in the toilsome letter, which is the “bread of anxious toil” (Ps. 127:2). He asks that this secret be removed and this hidden thing be brought forth into the light, speaking throughout the psalm in a relative way and speaking and distinguishing by way of comparison between the old and the new law.
Luther’s works, vol. 11: First Lectures 2