Read The Raging Battle, by DR Tony Evans of Alternative View, at One Place.
"The very Spirit of God has taken up residence in our lives if we have accepted Jesus Christ as our Savior. God wants to express His mind, His thoughts, His desires, and His will through our mortal bodies. But for us to see this happen, we need to be under the Holy Spirit’s control. While this is taking place, we are going to experience conflict within ourselves. This is one way we can know we are Christians. If we have no battles raging within, we are probably not saved. There is no opposition within us. God says the flesh and the Spirit are total opposites; they are not going to get along with each other at all."
But I say, walk by the Spirit, and you will not carry out the desire of the flesh.
For the flesh sets its desire against the Spirit, and the Spirit against the flesh;
for these are in opposition to one another, so that you may not do the things that you please.