John 3:9,10 - Coke's Commentary on the Holy Bible

Nicodemus answered, &c.— Our Lord's arguments were undeniable: nevertheless, Nicodemus, who had been accustomed to the pomp and ceremony of an external religion, surprised to hear that Jews (who by birth were the people of God,) must be begotten and born again, still urged that the thing was impossible; which it doubtless was, taking regeneration, as he did, for conversion to Judaism, a notion which he was led into from what Jesus had told him, John 3:5-6 namely, that the regeneration he spake of was a moral and spiritual one. Our Lord replies, Art thou a master,—διδασκαλος, a teacher in Israel,—and knowest not these things? Our Lord having all along spoken to Nicodemus in the common dialect of the Jewish divinity schools, might justly express his surprise, that he, who was a teacher in Israel, did not understand it: for though he affixed a meaning to the word regeneration different from what it bare in the mouths of the doctors, it was plainly analogous to their sense of it, and so might easily have been understood even by a novice; the admission of a proselyte being looked upon by the Jews as a second birth to him, in regard that his parents and relations were no longer reckoned such, and the proselyte was thought to have received a new soul by the change of his religion. It is strange that any should doubt whether proselytes were admitted into the Jewish church by baptism, that is, by washing; when it is plain, from express passages in the Jewish law, that no Jew who had lived like a Gentile for one single day, could be restored to the communion of their church, except by baptism. Compare Numbers 19:19-20 and many other passages relating to ceremonial pollutions, by which the Jews were rendered incapable of appearingbefore God in the tabernacle or temple, till they were washed, either by bathing or sprinkling.