Mortification of sin is to be the daily work of believers. "You must always be at it while you live," writes Owen. "Do not take a day off from this work; always be killing sin or it will be killing you" (p. 5)
Why is this so necessary? Chapter two gives six answers:
1. Indwelling sin always abides while we are in this world; therefore, there is always a need for it to be mortified.
2. Sin is still acting and labouring to bring forth the deeds of the flesh. "When sin lets us alone, we may let sin alone; but sin is always active when it seems to be the most quiet, and its waters are often deep when they are calm . . . . Sin is always acting, always conceiving, always seducing and tempting. Who can say that he has ever had anything to do with God or for God which indwelling sin has not tried to corrupt? . . . If sin is always acting, we are in trouble if we are not always mortifying. He that stands still and allows his enemies to exert double blows upon him without resistance will undoubtedly be conquered in the end. If sin is subtle, watchful, strong, and always at work in the business of killing our souls, and we are slothful, negligent, and foolish in this battle, can we expect a favourable outcome?" (p. 7).
3. Sin, if not continually mortified, will bring forth great, cursed, scandalous, and soul-destroying sins (Gal. 5:19-20). "Every time sin rises to tempt or entice, it always seeks to express itself in the extreme. Every unclean thought or glance would be adultery if it could; every covetouos desire would be oppression; and every unbelieving thought would be atheism. It is like the grave that is never satisfied" (p. 8).
4. The Holy Spirit and our new nature are given to us to oppose sin and lust (Gal. 5:17; 2 Pet. 1:4). "If we do not seek daily to mortify sin, we sin against the goodness, kindness, wisdom, grace, and love of God, Who has given us the weapons of our warfare" (p. 9).
5. Neglect of this duty makes the inner man to decay instead of renewing him. "When poor creatures will take blow after blow, wound after wound, foil after foil, and never rise up to a vigorous opposition, can they expect any thing but to be hardened through the deceitfulness of sin, and that their souls should bleed to death" (p. 9-10).
6. Our spiritual growth is our daily duty. (cf. 2 Cor 7:1; 1 Pet 2:2; 2 Pet 3:18; 2 Cor 4:16). "Sin sets its strength against every act of holiness, and every degree of spiritual growth. We will not be making progress in holiness without walking over the bellies of our lusts. He who does not kill sin along the way is making no progress in his journey" (p. 10).