The Apostle Paul writes about the “inward man” and the “outward man”; the “new creation” and the “man of flesh” (2 Corinthians chapters 4 and 5). It is a unique concept of spiritual treasure contained within vessels of clay; the eternal life of Christ in dying human bodies. See also 1 Corinthians 15:45-49 that speaks of bearing the “image of the earthy” and the “image of the heavenly”.
What Paul says in these few chapters demolishes human-centered religion with its focus on personal improvement to please God. The radical truth is that God does not want to improve our old self—our “old man”—He wants it to die! Jesus said as much: If any man will come after me, let him deny himself, and take up his cross [an instrument of death], and follow me. (Matthew 16:24).
When the natural self is the starting point of those striving to live a godly life, they will eventually give up trying to be good, or they will be increasingly unhappy in their striving because perfection is beyond human reach. We have all met Christians like that—either phony or miserable.
There is a better way. The essence of Christianity is God’s demand for perfection in His presence that requires us to hand our whole self over to Him so that He can make of us an entirely new self! C.S. Lewis notes that this is very hard to do but not nearly as hard as achieving perfection on our own. “In fact, it is impossible,” Lewis cautions, adding: “It may be hard for an egg to turn into a bird, but it would be a lot harder for it to learn to fly while remaining an egg.”