[Meditation.] Hence he aptly adds “they are my meditation.” For to meditate means to think deeply and to explore the inner parts and always to follow the spirit within and not to construct a wall for yourself and set up a boundary, as if you had already achieved the end of understanding or acting. Therefore I have rightly said that the testimonies require faith above all, so that in what you do not yet understand you believe one who understands and you do not by your own authority set up the meaning for yourself or fight against another in what you do not know or about which you have doubts. Therefore to meditate means to know the testimonies inwardly since they are the signs and attestations of things to come. He who does not understand the Scripture in a relative way or does not work toward the future, namely, that he always knows that there is something left over for him to understand and do and faithfully to await and desire finally to understand and do, he certainly does not let the Scripture be the testimonies of the Lord. But now, until the life to come, there is always something left over to understand and to do. Therefore you must never be proud, as if you were already full, rich, and well provided. Always they are to you testimonies of what you have not yet understood or done. And yet you ought to wish and expect to do and understand them, as above: “My soul has coveted to long for Thy ordinances at all times” (v. 20). Not that it is necessary for us to understand and do everything in this life, but that the mind should be prepared never to want to stop doing and understanding more fully to eternity, to know no boundary, no end, no restriction. This is what it means to be in the spirit of freedom, for which no law and statute has been set, because it does more than is commanded, so that if one could live eternally, one would strive eternally to know and do, and never move backward. He is the one whose meditation is the testimonies of the Lord and who keeps His testimonies.
Luther’s works, vol. 11: First Lectures 2