Nicodemus was an expert in what we would now call "Old Testament" scripture, including the law as well as the myriad of Jewish oral traditions and Pharisaical practices. What I’m saying is that Nicodemus was an intelligent, religious man who worked very hard at his faith.
Nicodemus was also smart enough to recognize that Jesus had something mysteriously profound to offer regarding spiritual insight. So Nicodemus approached Jesus one day. John chapter three shares this encounter for us. Nicodemus offers a respectful greeting… Jesus responds with a statement about being born again. Nicodemus responds dumbfoundedly… Jesus responds with further statements about being born again. Nicodemus responds again dumbfoundedly… Jesus responds with the verse held up by the guy in the rainbow wig at the football games. Basically, Jesus explains the necessity of conversion, the process of conversion, the purpose of conversion, the passion of conversion, and how conversion is possible. And Nicodemus doesn’t get it.
Now Jesus is dumbfounded. He is amazed that Nicodemus doesn’t understand the idea of conversion. "You are Israel’s teacher," he said to Nic, "and you don’t understand these things?"
I don’t think Jesus was slamming Nicodemus’ intellect. I do think Jesus was implying that Nicodemus, with all of his learning and training, should have understood these things! Nicodemus had read the Scriptures! Nicodemus had studied and trained under the brightest of Israel’s religious scholars and rabbis. He taught people everyday to live for God and obey the commands. And Jesus is dumbfounded that Nicodemus didn’t understand how God wanted to work in changing people’s lives.