Grace is poured upon Your lips. Experience has made this clear and continues to do so. For when Christ and the apostles preached invisible and incomprehensible things, which eye has not seen, etc., and for the sake of these ordered people to despise all visible things, good as well as evil, they would never have prevailed if there had not been an attracting grace in their word. Not so Moses or any human lawgiver. For they commanded only temporal things, and if they had commanded invisible things, they would never have been persuasive, because there would not be grace but rather indignation and wrath on their lips. Therefore here, too, the prophet said: “My tongue is the pen of a writer.” For the tongue of Moses was not such a pen when he spoke the letter. Therefore, when he urged something absent, future, or difficult upon the Jews in the wilderness, such as taking the enemies by storm or eating of meats, etc., there was no grace on his lips, because they were not moved except by the bare word. But in the new law my tongue, he says, which speaks spirit and whose speech is the utterance of the heart, is always covered with grace. For wherever the Word of the Spirit is preached, it is not preached without result. For “grace is poured upon Your lips.” Therefore a distinction must be made here: The lips of Christ transcend personal ones
First, by hearing and expounding both testaments spiritually.
Second, they are all the lips of his saints, those who are truly saints, even if they do not preach publicly.
Third, they are the lips of all who preach the Gospel or from whom the Gospel comes, that is, the spirits of the old law, even if they were not personally saints, especially when by virtue of their office they teach both by the church’s authority and publicly.
On all these lips grace is poured, for they preach such things that, unless cooperating grace would come to the hearer at the same time, they would achieve no results. But they do achieve results, because this prophecy was not spoken in vain. Therefore these pronouns in this psalm, your lips, your sword, your arrows, must always be referred to the spirit according to the instruction placed in the title. And thus their tongues are always the pen of a writer, for when they speak with that tongue, the Holy Spirit at the same time writes in the hearts of the hearers by means of it. Hence the corollary: We note that the Word of God is to be heard gladly, because it is never preached in vain. Is. 55:11: “The Word which goes forth from My mouth shall not return to Me empty.”
Luther’s works, vol. 10: First Lectures I