Nicodemus had obviously been listening to Jesus. But the more he thought he understood, the less he actually knew. Now, you wouldn't be able to tell that just from looking at him. Outwardly, Nicodemus is the picture of confidence and self-assurance. He's setting the ground rules for this conversation they're about to have.
Nothing is to be left to chance.
He knows about people, knows that they are born to grow old and die. But something, maybe something he heard Jesus say or saw Jesus do, has confused him, has caused him to wonder if he really understands everything he knows about God. Nicodemus came to Jesus that night looking for a formula, a tried and tested set of rules to add to the church's already lengthy list of rules. The writer of the Gospel of John portrays Nicodemus as a sincere man, a devout man, who obeys the law and exercises responsible leadership in his community. But at the level of faith, there is something tentative about Nicodemus.