The word flesh is often used in the Bible in reference to an unregenerated person. At the fall of man, the soul rejected the authority of the spirit and became enslaved to the body and its lustful desires. This is how man became fleshly. Since the spirit has lost its noble position and became a captive, and the soul is subjected to the power of the body, man is regarded in the Bible as having become flesh.
The Lord Jesus said, “That which is born of the flesh is flesh” (John 3:6). Whatever a man has derived from nature, when he was born of his parents is flesh. However good he is, virtuous he may be, whatever talents he may have, or however kind and intelligent he may be, he is fleshly. Regardless of how bad, how unholy, how foolish, how useless, or how cruel he may be, he is of the flesh. That man is flesh means that all that a man inherits by birth, whatever it may be (good or bad) is of the flesh. Man does not become fleshly by learning or practising to be bad.