John 3:7 - Ellicott's Commentary On The Whole Bible

Ye must be born again. — The laws of natural and spiritual generation have been stated as general truths, holding good for all mankind, “that which is born.” But there is a special application to the present case, “Marvel not that I said unto thee (teacher as thou art) that ye (children of Abraham as ye are) must be born again.” In so far as they were children of Abraham according to the flesh, they were children of Abraham’s physical and sinful nature. The law of that, as of all human nature, was that flesh ruled animal life, and animal life ruled spirit, and the whole man became carnal, bringing forth the fruits of the flesh. The law of the regenerate nature was that the spirit, born by the influence of the Divine Spirit, rose to a new life of communion with God, controlled the lower life, with its affections, feelings, and desires, and that these thus controlled became the motive power of the body; the whole man thus became spiritual, bringing forth the fruits of the Spirit. (Comp. Note on 1 Thessalonians 5:23.) For them, then, as for all, it was no matter of wonder, it was an absolute necessity of their true life, that they should be born anew.