While Scripture often uses the term flesh to describe the physical body, it is also used to portray that part of our nature which encompasses body, mind and soul. This definition of flesh is epitomized in Jesus’ warning to His disciples, “the spirit is willing but the flesh is weak.”(Matthew 26:41). Here flesh is used to describe that part of our nature which stands in contrast to our spiritual nature. It is the carnal nature of man, the seat of sinful passions and affections.
Our flesh, then, is that part of our nature which is able to live and function freely without any submission to God—although it’s obvious that it can exist only as His sustaining power allows it. And it’s very important to understand that the flesh follows us into our new life in God. It continues to have a life of its own even as our spirit is reborn and brought alive. As a result, we find ourselves constantly embroiled in a battle with the desires of our flesh as we seek to fulfill the desire of the Spirit.(Galatians 5:17 “For the flesh sets its desire against the Spirit, and the Spirit against the flesh; for these are in opposition to one another…”)
The Bible also uses another term for the flesh: the natural man. The term natural man is synonymous with the flesh—but only the flesh as it pertains to the unregenerate man. Once we have received Christ and become a child of God, though still saddled with the flesh, we are no longer a natural man but a spiritual one.(1 Corinthians 2:14-16 “But a natural man does not accept the things of the Spirit of God, for they are foolishness to him; and he cannot understand them, because they are spiritually appraised.15But he who is spiritual appraises all things, yet he himself is appraised by no one.16For who has known the mind of the Lord, that he will instruct Him? But we have the mind of Christ.”)
Excerpted from: Free from the Power of Sin: The Keys to Growing in God in Spite of Yourself