Adam Clarke on John 3:11

We speak that we do know - I and my disciples do not profess to teach a religion which we do not understand, nor exemplify in our conduct. A strong but delicate reproof to Nicodemus, who, though a master of Israel, did not understand the very rudiments of the doctrine of salvation. He was ignorant of the nature of the new birth. How wretched is the lot of that minister, who, while he professes to recommend the salvation of God to others, is all the while dealing in the meagre, unfruitful traffic of an unfelt truth! Let such either acquire the knowledge of the grace of God themselves, or cease to proclaim it.

Ye receive not our witness - It was deemed criminal among the Jews to question or depart from the authority of their teachers. Nicodemus grants that our Lord is a teacher come from God, and yet scruples to receive his testimony relative to the new birth, and the spiritual nature of the Messiah's kingdom.