From the Word
17 Now this I affirm and testify in the Lord, that you must no longer live as the Gentiles do, in the futility of their minds; 18 they are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart; 19 they have become callous and have given themselves up to licentiousness, greedy to practice every kind of uncleanness. 20 You did not so learn Christ! — 21 assuming that you have heard about him and were taught in him, as the truth is in Jesus. 22 Put off your old nature which belongs to your former manner of life and is corrupt through deceitful lusts, 23 and be renewed in the spirit of your minds, 24 and put on the new nature, created after the likeness of God in true righteousness and holiness.
25 Therefore, putting away falsehood, let every one speak the truth
Ephesians 4:17–25, RSV
The new man has the Spirit and the truth, by which the heart is illuminated unto righteousness and holiness wherein man follows the guidance of God’s Word and feels a desire for a godly walk and a good life. This new man is created after God, as an image of God, and must of necessity differ from such as live in error and in lusts, without the knowledge of God and disobedient to him. For if God’s image is in man, man must consequently have the right knowledge of God and right conceptions and ideas, and lead a godly life consistent with holiness and righteousness as found in God himself.
Such an image of God Adam was when first created. He was, as to the soul, truthful, free from error, possessed of true faith and knowledge of God; and as to the body, holy and pure, that is, without the impure, unclean desires of avarice, lasciviousness, envy, hatred and the like. Thus the whole life of the man was a beautiful portrait of God, a mirror wherein God himself was reflected; even as the lives and natures of the holy spirits and angels are wrapped up in God and represent true knowledge of him, assurance, and joy in hm and utterly pure and holy thoughts and works according to the will of God.
All Adam’s children — all men — would have so remained from their birth, if Adam had not suffered himself to be led astray by the devil and to be thus ruined. But since Christians, by the grace and Spirit of God, are now renewed in this image of God, they are so to live that soul and spirit are righteous and pleasing to God through faith in Christ; and that also the body — meaning the whole external life — be pure and holy. They thus enter again into their former relation and into the true paradise of perfect harmony with God and of justification; they are comforted by his grace. They are disposed to lead a godly life and resist ungodly lusts and ways. They begin to taste God’s goodness and lovingkindness and to realize what they lost in paradise. He, therefore, that would be a Christian should strive to be found in this new man created after God — in the very essence of righteousness and holiness before God.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 365–66.