That which is born of the flesh, is flesh
Man by his natural birth, and as he is born according to the flesh of his natural parents, is a mere natural man; that is, he is carnal and corrupt, and cannot discern spiritual things; nor can he, as such, enter into, and inherit the kingdom of God; see ( 1 Corinthians 2:14 ) ( 15:50 ) . And therefore there is a necessity of his being born again, or of the grace of the Spirit, and of his becoming a spiritual man; and if he was to be, or could be born again of the flesh, or ever so many times enter into his mothers womb, and be born, was it possible, he would still be but a natural and a carnal man, and so unfit for the kingdom of God. By "flesh" here, is not meant the fleshy part of man, the body, as generated of another fleshy substance; for this is no other than what may be said of brutes; and besides, if this was the sense, "spirit", in the next clause, must mean the soul, whereas one soul is not generated from another: but by flesh is designed, the nature of man; not merely as weak and frail, but as unclean and corrupt, through sin; and which being propagated by natural generation from sinful men, cannot be otherwise; for "who can bring a clean thing out of an unclean? not one", ( Job 14:4 ) . And though the soul of man is of a spiritual nature, and remains a spirit, notwithstanding the pollution of sin; yet it being defiled with the flesh, and altogether under the power and influence of the lusts of the flesh, it may well be said to be carnal or fleshly: hence "flesh", as it stands opposed to spirit, signifies the corruption of nature, ( Galatians 5:17 ) ; and such who are in a state of unregeneracy, are said to be after the flesh, and in the flesh, and even the mind itself is said to be carnal, ( Romans 8:5-8 ) .
And that which is born of the Spirit, is spirit:
a man that is regenerated by the Spirit of God, and the efficacy of his grace, is a spiritual man; he can discern and judge all things of a spiritual nature; he is a fit person to be admitted to spiritual ordinances and privileges; and appears to be in the spiritual kingdom of Christ; and has a right to the world of blessed spirits above; and when his body is raised a spiritual body, will be admitted in soul, body, and spirit, into the joy of his Lord. "Spirit" in the first part of this clause, signifies the Holy Spirit of God, the author of regeneration and sanctification; whence that work is called the sanctification of the Spirit, and the renewing of the Holy Ghost, ( 1 Peter 1:2 ) ( Titus 3:5 ) . And "spirit", in the latter part, intends the internal work of grace upon the soul, from whence a man is denominated a spiritual man; and as a child bears the same name with its parent, so this is called by the same, as the author and efficient cause of it: and besides, it is of a spiritual nature itself, and exerts itself in spiritual acts and exercises, and directs to, and engages in spiritual things; and has its seat also in the spirit, or soul of man.