At any given moment, a Christian is living in one of two modes: in the flesh or in the Spirit... according to our corrupt nature, or according to our spiritual nature. There's nothing in the middle, although I suppose the complexity of life and the reality of compromise make it seem like there is sometimes. Scripture tells us in Romans that we have a choice:
If you live according to the flesh, you will die; but if by the Spirit you put to death the deeds of the body, you will live. (Romans 8:13 NRSV)
Dang, Paul. Since you put it like that...
I get the feeling from this verse that there's more to Christianity than altar calls and fire insurance. Are we telling those new converts what they're getting themselves into?
Let's make something clear... the body isn't evil. That's not what "flesh" is referring to here. But there is something about our human nature that has been corrupted by sin... it's like a default setting. We're somewhat like a car that's badly out of alignment. If the driver lets go of the steering wheel in such a car for a few seconds, that car will likely run off the road- even if the road is a straight one. In the same way, without an infusion of transforming power from the Holy Spirit (a realignment, if you will) and our regenerated self at the wheel, we're not going to stay on a straight path.
I like the way God's Word translates flesh in the passage I quoted above:
If you live by your corrupt nature, you are going to die. But if you use your spiritual nature to put to death the evil activities of the body, you will live. (Romans 8:13 GW)
In other words, even as Christians, if we let our flesh do what it wants, things aren't going to turn out well. Scripture makes it pretty clear that sin brings death. But which kind of death? Spiritual or physical? Perhaps we should read this passage on both levels. Plenty of people (Christians included) have probably shortened their days on earth because they couldn't get a handle on their flesh. And on the spiritual side, there's backsliding. While I think it's very difficult for a Christian to walk away from God and become lost, I do believe the possibility exists. Sinning dulls our consciences and when we stubbornly continue to resist the conviction of the Holy Spirit, I believe we're in dangerous territory. That doesn't mean God won't pull out all the stops to get us back on the right track... but ultimately he honors our free will.
The Holy Spirit offers amazing power to help us live the Christian life. God wants us to move on to perfection... but it isn't automatic. Choices we make affect who we become, and even as Christians, we have free will. We can choose to walk in the flesh or walk in the Spirit, and each day is filled with opportunities to make that choice.