John 3:6

John 3:6. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.

So that the best child who ever was born, even though he were, like Saul of Tarsus, "of the stock of Israel, of the tribe of Benjamin, a Hebrew of the Hebrews;" yet even he, inasmuch as he "is born of the flesh, is flesh," and not "spirit." Everything which comes to us by our first birth can be nothing better than flesh, and what can you get out of flesh but flesh? The only "evolution" that can come of the flesh is corruption. There must be another birth if you are to get anything but flesh: "that which is born of the Spirit is spirit." Fleshly things are understood by the flesh, and spiritual things must be spiritually discerned; hence the absolute necessity of a second birth, a Spirit birth, that we may first see and then enter the spiritual kingdom of God.