Here is Martin Luther, in his own words, on Justifying Faith and Sola Fide; framed ostensibly in his famed Law versus Gospel nexus as constituted in the Word. He says:
From all this it is easy to perceive on what principle good works are to be cast aside or embraced, and by what rule all teachings put forth concerning works are to be understood. For if works are brought forward as grounds of justification, and are done under the false persuasion that we can pretend to be justified by them, they lay on us the yoke of necessity, and extinguish liberty along with faith, and by this very addition to their use they become no longer good, but really worthy of condemnation. For such works are not free, but blaspheme the grace of God, to which alone it belongs to justify and save through faith. Works cannot accomplish this, and yet, with impious presumption, through our folly, they take it on themselves to do so; and thus break in with violence upon the office and glory of grace.
We do not then reject good works; nay, we embrace them and teach them in the highest degree. It is not on their own account that we condemn them, but on account of this impious addition to them and the perverse notion of seeking justification by them. These things cause them to be only good in outward show, but in reality not good, since by them men are deceived and deceive others, like ravening wolves in sheep’s clothing.
Now this leviathan, this perverted notion about works, is invincible when sincere faith is wanting. For those sanctified doers of works cannot but hold it till faith, which destroys it, comes and reigns in the heart. Nature cannot expel it by her own power; nay, cannot even see it for what it is, but considers it as a most holy will. And when custom steps in besides, and strengthens this pravity of nature, as has happened by means of impious teachers, then the evil is incurable, and leads astray multitudes to irreparable ruin. Therefore, though it is good to preach and write about penitence, confession, and satisfaction, yet if we stop there, and do not go on to teach faith, such teaching is without doubt deceitful and devilish. For Christ, speaking by His servant John, not only said, “Repent ye,” but added, “for the kingdom of heaven is at hand” (Matt. iii. 2). (quote taken from here)
Notice the last paragraph, . . . Nature cannot expel it by her own power; nay, cannot even see it for what it is, but considers it as a most holy will. And when custom steps in besides, and strengthens this pravity of nature, as has happened by means of impious teachers, then the evil is incurable, and leads astray multitudes to irreparable ruin. . . ., what he is getting at is the scholastic/thomistic notion of “nature being perfected or completed by grace; he believes that what is required is not a rebuilding of nature (man), but a recreating of nature in Christ. Whether the Holy Spirit, who is uncreated comes to the soul of man, and simultaneously provides that soul with created grace, or whether He the Spirit so “deifies” nature enabling it to “cooperate” with grace climaxing in condign merit, eternal life . . . whatever language one wants to use, Luther is repudiating this notion, along with St. Paul. There is nothing repairable about nature (see Rom 8:3), for Luther, and Protestants grace is Christ applied as donum, via the Holy Spirit in the heart of man. Nature is radically and apocalyptically recreated in Christ. In this sense, Luther speaks of “justifying faith” which results in union with Christ, eschatologically . . . once union is achieved with Him we are forever free, and guaranteed (cf. II Cor. 5:5) for consummation with our beautiful bride-groom, Jesus Christ.