6 That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.
This verse points to the difference between physical and spiritual birth. It is a principle that these two spheres are fundamentally different.
6 That which is born of the flesh is flesh,
“Flesh” refers here to the physical body and human nature. Only human beings can beget human beings. Like generates like. Physical life produces physical life, and the Spirit generates spiritual life.
and that which is born of the Spirit is spirit.
Only the Spirit can beget spiritual beings. Those who experience spiritual birth do so by the work of the Holy Spirit. The Spirit transforms the nature of those who believe in Christ; a born-again person has a new nature. Being born of the Spirit is the central essential to possess eternal life.
Physical birth and spiritual birth are two different and distinct realms.
Salvation is not an addition to physical life; it is a brand new beginning of life with God, a spiritual birth. Physical life and spiritual life are two different spheres of reality. We cannot receive spiritual life by any human system. Salvation is not contingent on human faculty or effort. It is necessary to humbly accept our spiritual bankruptcy (Php 3:8-9).
Spiritual life differs from physical life not simply in the material and non-material sense but in the meaning of mortal versus immortal. God is immortal and we receive His eternal life at salvation. We share His immortal life.
Salvation is not something we produce or manufacture. It is what we undergo. It is a birth that produces regeneration, new life. Flesh can produce only flesh, nothing higher. People cannot come to God by their own effort. Men can produce babies but nothing else. This is axiomatic. Like produces like; flesh engenders flesh. A stream never rises higher than its source. A dog cannot produce kittens. Man can produce only what is natural to man. That is why he is sinful by nature.
Salvation cannot come by outward conformity; it is the Holy Spirit who produces it. Man moved by his degenerate nature cannot come to God. All we are as a result of our natural birth is our sinful nature. This is a mighty handicap.
God does not save us based on anything in man. It is possible to polish our lives with religion, but this is a façade. We can paint a pump over a well of impure water, but it will not purify the water. No man can come to God by human effort. Flesh produces only flesh. Our new nature is the only aspect of us that is of any good. That nature is incapable to sin.