Strictly, that which hath been born, and consequently is now before us as born. The aorist tense (Joh 3:3, Joh 3:4, Joh 3:5, Joh 3:7), marks the fact of birth; the perfect (as here), the state of that which has been born (see on 1Jo 5:18, where both tenses occur); the neuter, that which, states the principle in the abstract. Compare Joh 3:8, where the statement is personal: everyone that is born. Compare 1Jo 5:4, and 1Jo 5:1, 1Jo 5:18.
Of the flesh (ἐκ τῆς σαρκὸς)
See on Joh 3:14. John uses the word σάρξ generally, to express humanity under the conditions of this life (Joh 1:14; 1Jo 4:2, 1Jo 4:3, 1Jo 4:7; 2Jo 1:7), with sometimes a more definite hint at the sinful and fallible nature of humanity (1Jo 2:16; Joh 8:15). Twice, as opposed to πνεῦμα, Spirit (Joh 3:6; Joh 6:63).
Of the Spirit (ἐκ τοῦ πνευματος)
The Holy Spirit of God, or the principle of life which He imparts. The difference is slight, for the two ideas imply each other; but the latter perhaps is better here, because a little more abstract, and so contrasted with the flesh. Spirit and flesh are the distinguishing principles, the one of the heavenly, the other of the earthly economy.