‘ Jesus answered and said to him, “Are you the teacher of Israel, and do not understand these things?” '
Jesus words are a gentle rebuke. ‘Do you claim to be a teacher (literally ‘the master' - therefore a particularly learned teacher) of Israel and yet do not understand this?' Many Pharisees were seen as ‘teachers of Israel', and Nicodemus was particularly highly respected. They thought that it was from their teachers that men must find the secrets of God. So Jesus wants him to know that He considers that he should understand it because he has seen it in action in Himself and John the Baptiser. There are ‘earthly things' witnessed on earth through the successes of John the Baptiser and the successes of His own ministry. Nicodemus really ought to have understood. But there are none as blind as those who are too certain that they are themselves right.