The nature of sin

2 As for you, you were dead in your transgressions and sins, ² in which you used to live when you followed the ways of this world and of the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient.

Here, Paul describes the nature of sin. Which he describes as spiritual. We see it in the phrase "and of the ruler of the kingdom of the air". This reference is to the devil. How many times do we view sin as just "another problem" that is when we do not recognize that there is a spirit behind it. Behind the sin is a deeper revelation that the Devil is behind this. When we do not recognize that then we would be fighting the war with our bare hands, alone. Without the help of God. But when we recognize that this is the work of the devil, then only then we can summon God's strength to fight this war. 

Then when the war is won then we must not forget Christ. If only the war was win by Christ.

³ All of us also lived among them at one time, gratifying the cravings of our flesh^[a] and following its desires and thoughts. Like the rest, we were by nature deserving of wrath. ⁴ But because of his great love for us, God, who is rich in mercy, ⁵ made us alive with Christ even when we were dead in transgressions—it is by grace you have been saved.

And then we are saved by Christ. Only Christ. It is by the grace of God, not your own strength, but by the grace of God. Remember these sins not to commit them again but to testify for His name. Only for his Glory. If it was fought by your own strength what victory would that be? The battle was never on the surface but in spirit. And only God can win this war in the spiritual realm, but we must be willing.