According to the express teaching of Holy Scripture it is impossible for fallen man to satisfy the demands of divine justice and to atone for his transgressions by good works, Ps. 49, 7. 8; Matt. 16, 26. On the contrary, all who seek to appease God by the works of the Law remain under the curse and condemnation of the divine Law, Gal. 3, 10. In fact, man by nature is so blinded .and corrupted by sin, 1 Cor. 2, 141; Eph. 2, 1, that his carnal heart is enmity against God, Rom. 8, 7, and therefore unable rightly to love and worship Him, 1 Cor. 10, 20; Eph. 2, 12. Man by nature is thus incapable of saving himself, Rom. 3, 10-20.
However, what man was unable to do God in His infinite mercy has accomplished for him, Rom. 8, 3. 4. Through the most perfect obedience of His beloved Son, Gal. 4, 4. 5 ; Is. 53, 4-6, He has reconciled the world unto Himself, 2 Cor. 5, 19; 1 John 2, 2, and since He has blotted out the handwriting of the Law which was against sinful mankind, Col. 2, 13. 14, He now offers to all sinners the merits of Christ through the means of grace (the Gospel and the Sacraments), earnestly desiring (vocatio seria) that all men should accept the most gracious forgiveness which He offers in Christ Jesus, 2 Cor. 5, 20. 21. This is the Scriptural basis of the doctrine of conversion. In other words, conversion is possible only because Christ by His suffering and death has secured salvation for lost mankind, John 1, 29, and because God in His indescribable grace offers this salvation to all sinners as a free gift, Eph. 2, 8. 9.
Read also Scriver's Seelen-Schatz in Google Books or in the browser where he treats the doctrine of repentance/ conversion in hundreds of pages (the second book of Seelen-Schatz).
Luther hardly treats Hos. 2: 6-7 or the Parable of the Prodigal Son (which is strongly linked to the doctrine of repentance/ conversion). But in two sermons Luther mentions the prodigal son, which should be mentioned here because it is related to conversion:
(5) All beginnings are difficult, but once the beginning is made, everything else becomes easier and comes as it were of its own accord. Just as, when something is to be set in motion, the impetus requires the most strength, but in the midst of the motion the thing runs by itself, so also he who begins to fight against his evil desires finds much work in the beginning, but in time the overcoming becomes easier and easier for him. That is why the prophet says: "Turn to me, and I will turn to you. Mal. 3, 7; as if he wanted to say: Seek me, and I will also go to meet you; just as that father hurried to meet his prodigal son when he set out to meet him, Luc. 15, 20.
(6) These sayings are therefore not to be understood of the first grace alone, but of both graces. The first grace causes man to make up his mind to turn to God, and of this it is said: "Turn to me. But the other grace makes man perfect in God, and of this it is said, "I will turn to you." I say this because such sayings, and some others like them are equal to some, make much work in their minds; for they deal with the increasing grace, but they understand the same from the beginning grace."Luther's Works: Catechetical writings and sermons. Of the ten commandments in particular. First commandment. Of true worship. Sermon on the fear of God.
"Turn to me, and I will turn to you"; as if he wanted to say: Seek me, and I will meet you. Just as the Father went to meet the Prodigal Son while he was still far away. Hence those sayings themselves are to be understood not of the first grace alone, but of both graces. The first grace is the one that makes a man turn to Him, of which it is said: "Turn to Me" Zech. 1:3. The other grace is that which makes man perfect, of which it is said, "And I will make him perfect."