From the Word
6 We know that our old self was crucified with him so that the sinful body might be destroyed, and we might no longer be enslaved to sin. 7 For he who has died is freed from sin. 8 But if we have died with Christ, we believe that we shall also live with him. 9 For we know that Christ being raised from the dead will never die again; death no longer has dominion over him. 10 The death he died he died to sin, once for all, but the life he lives he lives to God. 11 So you also must consider yourselves dead to sin and alive to God in Christ Jesus.
Romans 6:6–11, RSV
Man is here called old, not because of his years; for it is possible for a man to be young and strong and vigorous and yet to be without faith or a religious spirit, to despise God, to be greedy and vainglorious, or to live in pride or the conceit of wisdom and power. But he is called “the old man” because he is unconverted, unchanged from his original condition as a sinful descendant of Adam. The child of a day is included as well as the man of eighty years; we are all thus from our mother’s womb. The more sins a man commits, the older and more unfit he is before God. This old man, says Paul, must be crucified — utterly condemned, executed, put out of the way, even here in this life. For where he still remains in his strength, faith and spirit cannot be; man remains in his sins, drowned under the wrath of God, troubled with an evil conscience which condemns him and keeps him out of God’s kingdom.
The “new man” is one who has turned to God in repentance, who has a new heart and understanding, and who, through the power of the Holy Spirit lives in accordance with the Word and will of God. It begins in baptism or in repentance and conversion. It resists and subdues the old man and its sinful lusts through the power of the Holy Spirit.
Although the “old man” is crucified in those who are new men, yet there remains in them in this life “the body of sin.” By this we understand the remaining lusts of the “old man,” which are still felt to be active in the flesh and blood, and which would fain resist the Spirit. But inasmuch as the head and life of sin are destroyed, these lusts cannot harm the Christian. Still he must take care not to become obedient to them, lest the “old man” come to power again. The new man must keep the upper hand; the remaining lusts must be weakened and subdued. And this body of ours must finally decay and turn to dust, thereby utterly annihilating sin in it.
Luther, Martin, and John Sander. Devotional Readings from Luther’s Works for Every Day of the Year. Augustana Book Concern, 1915, pp. 253–54.