Gal 6:17 For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
In the above page from Morning and Evening, Charles H. Spurgeon gives a very good overview of the Christian’s daily spiritual battle with the flesh (also called the natural man).
This battle takes place because our salvation is threefold: Judicial, Spiritual, and Physical.
Salvation is judicial in that we have imputed righteousness. We are given the righteousness of Christ. Salvation is spiritual in that we are born again, and are sealed by the Holy Ghost. The Holy Spirit dwells within the Christian and we are a new creature. Thus, two of the three parts of our salvation are accomplished.
The third part of our salvation, the physical, yet remains to be completed. In other words, one day we will receive a glorified body. Our old body is not good enough for heaven or eternity. We will get a body like Christ’s. As a result, because our physical has yet to be “fixed”, our new nature (the spiritual) and the old nature (the flesh) lusteth against each other.