In my small group last week, we got into a discussion on the nature of the new birth. The question arose, Is it possible for the Christian not to ever sin? Several of us replied yes, it is theoretically possible. We based that on the fact that as Christians we have "died to sin" - that is, died to its power over us as well as its condemnation of us.
Romans 6 teaches this. Paul says, "We were therefore buried with [Christ] through baptism into death in order that, just as Christ was raised from the dead through the glory of the Father, we too may live a new life" (Romans 6:4). Paul goes on to say that "sin shall no longer be your master" (Romans 6:14). Because of our union with Christ in His death, burial, and resurrection, we have been "set free from sin" (Romans 6:7). At the cross, the flesh has been mortally wounded; "the body ruled by sin [has been] done away with" (Romans 6:6). We have been "brought from death to life" (Romans 6:13).
What this means practically is that when tempted to sin, I can look sin in the eye and say, "Sin, you do not tell me what to do. You are no longer my master. The risen life of Christ now defines me, you don't. The person I have become in Christ is not your slave; rather, you are my slave. I say to you, be gone. I count myself dead to you and alive to God."
Theologians since the time of Augustine have talked about the four-fold state of man. At creation, Adam had both the power to sin and the power to not sin (posse non peccare et posse peccare). After the fall in the Garden of Eden, man lost the will to obey God and became unable not to sin (non posse non peccare). However, when a person is regenerated by the Holy Spirit, he or she becomes able again not to sin (posse non peccare). While that is a wonderful thing, it's not the ultimate blessing. The ultimate blessing is not being able to sin (non posse peccare) - a blessing reserved for us in heaven. (Note that our state in heaven will be better and more glorious than the state Adam was in before he fell!)
So the believer is able, by grace through faith, not to sin.
However, an honest Christian will say this is not his or her daily experience. It is certainly not mine. I have sinned and do sin a lot, and I know I'm a believer in Christ! The Bible even says in 1 John 1:8, "If we claim to be without sin, we deceive ourselves and the truth is not in us."
Interestingly, the same Paul who wrote Romans 6 also wrote Romans 7, in which he confessed, "I do not do the good I want to do, but the evil I do not want to do - this I keep on doing" (Romans 7:19). Paul experienced a daily struggle with indwelling sin. Some of the battles with sin he won, but others he lost. He had to admit that even after regeneration he was "a wretched man" in need of the grace of God (Romans 7:24-25).
So even though it is theoretically possible, in a moment-by-moment sort of way, to not bow to the temptation to sin, the fact is we do not actually reach a state of sinless perfection. And I would add that people who claim to have reached that point do not know themselves very well at all, and are probably extremely guilty of the sin of pride.
The vestiges of sin must be engaged in battle and fought with determination each and every moment. I must "put to death" those ugly traits that used to characterize me (Colossians 3:5). I must flee my idols (1 John 5:21). I must not "indulge the sinful nature" (Galatians 5:13). In my struggle with sin I cannot just "let go and let God," as many put it. Instead, the flesh is still so strong in me that I must arm myself with the arsenal of God and throw myself into the fight with everything I've got.
But there's a practical beauty to this idea that as a converted person I am "able not to sin." It gives me power to resist sin. It is the gospel I preach to myself in the heat of temptation. I find that it frequently kills the power of sin when I remind myself, "I have been crucified with Christ and I no longer live, but Christ lives in me. I have been raised up with Christ and seated with Him in the heavenly realms in Christ Jesus. Sin is no longer my master, because I am not under the law, but under grace" (Galatians 2:20, Ephesians 2:6, Romans 6:14).