I find there is a general misconception among modern Christians, as to the nature of sinfulness in and around us. Though Scripture clearly warns us of the power and intentions of the Enemy (1 Peter 5:8), many seem to ignore or trivialize this threat.
Be wary of the nature of your fallen heart; it is idolatrous by nature, ever seeking a master to serve. Rarely, if ever, does the heart incline towards God on its own. (I think in this instance, it is helpful to think of salvation as a response to the Glory of God, rather than an activity by which we move ourselves from a place of sinfulness into a place of godliness. It is God who calls, and it is God who saves, not us.)
It instead inclines towards worldliness, gently coaxed by the devil. Too often we live with a false confidence that God is our Lord, when we actually serve something else entirely. It is this subtle self-deception, rooted in false confidence, that slowly kills us from within. I think there there are certain Biblical and theological realities that can help us fight this quiet lurking sin.
I think the most important to be this: Do not hide behind the cross! Though satan was finished at Calvary, he remains alive until the second coming. Thus, Jesus’ death was both the end of the war and the beginning of the battle. Yes indeed, sin is vanquished! But we must daily take up our cross and follow Him. It is not enough to say, “Death has been beaten!”, and then live a life of spiritual complacency. Stand up! Wage war on the darkness that most certainly still pervades our world. Fight to dispel sin, pursue godliness. If you say this is difficult or impossible for us to do, your are correct (Read John 15). But see what God has done, His provisions are perfect, cling to Him, and He will keep you.
The salvation of the righteous is from the LORD; He is their stronghold in times of trouble. The LORD helps them and delivers them; He delivers them from the wicked and saves them, because they take refuge in Him. Psalm 37:39-40