Albert Barnes Commentary John 3:4

"Nicodemus saith unto him, How can a man be born when he is old? can he enter a second time into his mother`s womb, and be born?" — John 3:4 (ASV)

How can a man? etc. It may seem remarkable that Nicodemus understood the Savior literally, when the expression to be born again was in common use among the Jews. For them, it denoted a change from Gentilism to Judaism, achieved by becoming a proselyte through baptism. The term, in their usage, signified a transition from the state of a heathen to that of a Jew.

However, they never applied this term to a Jew, because they supposed that by birth every Jew was entitled to all the privileges of the people of God. Therefore, when our Savior used it in reference to a Jew, and affirmed its necessity for every man, Nicodemus perceived an absurdity in the doctrine, or something that surpassed his comprehension. He consequently asked whether it was possible that Jesus could teach so absurd a doctrine, since he could conceive no other meaning applicable to a Jew than that an old man should enter a second time into his mother's womb and be born. From this, we may learn the following:

1. That prejudice leads people to misunderstand the plainest doctrines of religion.
2. That things which are at first incomprehensible or apparently absurd may become clear when explained. The doctrine of regeneration, so difficult for Nicodemus, is plain to a child that is born of the Spirit.
3. Those in high rank and the learned are often most ignorant about the plainest matters of religion. It is often astonishing that they show so little familiarity with the simplest subjects concerning the soul, and so much absurdity in their views.
4. A doctrine is not to be rejected because the rich and the great do not believe or understand it. The doctrine of regeneration was not false because Nicodemus did not comprehend it.