This mortification is both a gift (of the Holy Spirit) and a duty (ours). In our own strength we cannot accomplish any lasting mortification, without the Spirit’s grace. But by the powerful and enabling grace of the Holy Spirit, we may and must hate sin, strangle it, and put a sword through it. We must meditate often on the horrific consequences of sinning against our beloved, triune God and Savior. We must know our own hearts and weaknesses, and avoid those situations that tend to promote the temptations that we are weakest in battling against. We must cast off all remnants of the life we left behind when we began to follow Christ. We must put ourselves under the death-dealing power of the cross of Christ (Gal. 6:14) so that the Spirit of Christ may put to death what is earthly in us.
Sin may assail but cannot master us, so long as we stand firm in Christ, calling upon His name. In Christ we are assured of God’s help in striving against sin. Though we may fall and lose various skirmishes against sin, because of our union and communion with Christ we have by faith the promise of ultimate victory and final deliverance, which, more than anything else, gives us hope and sustenance in the daily fight against sin. The only sin fatal to our cause is unbelief. Unbelief alone can rob us of God’s grace and shut us out of His kingdom.
(Joel Beeke, excerpts from an article published on Tim Challies’ blog)