John Lightfoot on John 3:4

[Can he enter the second time into his mother's womb?] The common opinion of the Jews about the qualification of an Israelite, qua Israelite, still sticks in the mind of this Pharisee: and although our Saviour useth that term, which in the Jewish language plainly enough intimates the necessity of being born from heaven, yet cannot he easily get off from his first prejudice about the Israelitish generation: "Whereas the Israelites, as they are Israelites, have a right to be admitted into the kingdom of the Messiah, do you therefore mean by this expression of yours, that it is necessary for any to enter a second time into his mother's womb, that he may be an Israelite anew?"

He knew and acknowledged, as we have already said, that there must be a sort of a new birth in those that come over to the Jewish religion; but he never dreamt of any new proselytism requisite in one that had been born an Israelite. He could not therefore conceive the manner of a new birth, that he should be made an Israelite anew, unless it were by entering into the mother's womb a second time; which to him seemed an impossible thing.