Through faith in Christ the believer becomes a new creature, Eph. 4, 24; Col. 3, 10; 2 Cor. 4, 16; 5, 17, who consents to God's will, Rom. 7, 22, and lives wholly unto God, in the newness of the spiritual life into which he has entered, Rom. 6, 1-11.
Nevertheless, while the believer thus serves God according to the inward, or new, man which has been implanted in him i t his conversion, Eph. 4, 24; Rom. 7, 22. 25, there remains in him also the old man, or the corruption of his nature, Eph. 4, 22; 2 Cor. 4, 16; Rom. 6, 6; 7, 18, so that according to the old man he is subject to sin, Rom. 7, 18-24, and continually opposes, and struggles against, the Spirit, Gal. 5, 17: ἡ σὰρξ ἐπιθυμεῖ κατὰ τοῦ πνεύματος.
Sanctification therefore occurs in the believer
The "old man" is the corrupt nature, or mind; the "new man," the mind conformed to God's will. While the believer is perfectly holy in so far as he is a new man, Rom. 6, 1-11; Eph. 4, 24; 1 John 3, 9, the old man is and remains thoroughly corrupt, Rom. 7, 18. Sanctification is effected not by reforming, Rom. 8, 13; Gal. 5, 24, but by crucifying and mortifying him, Matt. 18, 8. 9.
With regard to the struggle of the spirit against the flesh the Christian must note the following: -
a. The constant struggle between the two natures in the believer does not prove that he has fallen from grace, as many true Christians in hours of trial are inclined to think, but, on the contrary, it is a proof that he is living in the state of grace, Rom. 7, 22-25. There is spiritual death only when the struggle against the flesh has ceased, Rom. 8, 13.
b. Since the old man in the believer always remains corrupt, so that according to the flesh Christians are no better than unbelievers, who have never been born again, John 3, 5. 6, the believer must not bt surprised if he is tempted by his carnal nature to commit even gross sins, Rom. 7, 18; 1 Thess. 4, 3-7. On the other hand, this fact should induce him to continue without ceasing to mortify the deeds of his body and to crucify the flesh, Rom. 8, 13; Gal. 5, 24; Col. 3, 5; 1 Cor. 9, 27; Matt. 18, 8. 9.
c. The struggle against the carnal nature is both difficult and painful, since it is directed against the believer's own evil flesh, Heb 12, 1. Yet the good fight of faith against the flesh must go on t .• the end, 1 Tim. 6, 12; 2 Tim. 4, 7. It is of great comfort to believers that even the greatest saints in the Bible were obliged continually to wage war against their evil flesh, Rom. 7, 24.
d. Scripture assures the believers that in their struggles against the flesh they will finally obtain the victory provided they adhere to G'~d's Word and thus permit the Holy Spirit to work effectually in their hearts John 15, 7. 8; Eph. 6, 17; Rom. 8, 37; Luke 18, 26. 27; 2 Cor. 12, 10; 4, 8ff.; etc. It is understood that with such steadfast use of God's Word there must be joined also ceaseless, fervent prayer, Matt. 26, 41; Eph. 6, 18.
e. It is an important rule of Christian combat always to substitute for the evil impulse and desire of the flesh the corresponding holy impulse and desire of the inward man. In other words, when the Christian is tempted to murmur against God, let him praise and give thanks; when he is troubled with impure thoughts, let him strive the more after that chastity which the holy Savior demands of His disciples; if he is weary in well-doing, let him follow the more zealously the goal of goodness which is set before him, etc. However, to accomplish this, he must know the Word of God and like Christ meet every temptation to evil with proper passages from Scripture, Matt. 4, 1-11.
In conclusion, we may add what Luther says on this important matter (Holman transl., Vol. III, p. 31): "This life is not righteousness, but growth in righteousness; not health, but healing; not being, but becoming; not rest, but exercise. We are not yet what we shall be, but we are growing toward it; the process is not yet finished, but it is going on; this is not the end, but it is the road; all does not yet gleam with glory, but all is being purified." "Dass also dies Leben nicht ist eine Froemmigkeit, sondern ein Frommwerden, nicht eine Gesundheit, sondern ein Gesundwerden, nicht ein Wesen, sondern ein Werden, nicht eine Ruhe, sondern eine Uebung: wir sind's noch nicht, wir werden' s aber. Es ist noch nicht getan und geschehen, es ist aber in Gang und Bchwang. Es ist nicht das Ende, es ist aber der Weg; es gluehet und glaenzt noch nicht alles, es fuegt sich aber alles."