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?

Nicodemus the teacher of Israel appeared here in a very obtuse and unspiritual frame of mind, in that he ridiculed the Lord's requirement of a new birth. Even ordinary spiritual perceptiveness would have saved him from such a reply as this, which was merely another way of saying, "What you ask is an impossibility!" However, it was not so much the impossibility of a new birth that Nicodemus rejected, as it was the idea that such a thing was necessary. Nicodemus was a Pharisee, one of a class that had rejected out of hand the baptism of repentance for the remission of sins preached by John the Baptist (Luke 7:30). It should be remembered that John's baptism was from God, and that all who rejected it rejected God. This fact underlies the truth that the publicans and harlots entered God's kingdom before the Pharisees. They accepted John's baptism; the Pharisees did not. Christ and his apostles accepted the baptism of John and submitted to it; and that baptism was intended as preparatory for the kingdom of heaven; and, therefore, it is impossible to suppose that Nicodemus should have been excused for not knowing what Jesus meant by being "born of water," mentioned in the next breath: the excuse for Nicodemus being founded upon the sophistry that the baptism of the great commission was not announced by Jesus until long after this interview; but there was another water baptism much nearer at hand, of which Nicodemus did know, and which he had rejected along with others of his class. Again from Hovey:

The metaphor of the new birth appears to have been used by the Rabbis to describe the religious change in a Gentile who became a proselyte to Judaism; and the import of baptism as administered by John implied the same view of repentance, namely, that it was a burial of the old life, and entrance upon a new life.[4]