Jesus was giving witness of the things in heaven and the evidence of the kingdom of God being at hand.  Nicodemus acknowledges that Jesus must be a man of God because of the great miracles and understanding of God’s word in the Torah.  But Nicodemus does not accept the words of what He has witnessed of, and about God, and the kingdom of God.  This is why Nicodemus came to Jesus in the night.  Because he had fear of being discovered and his reputation in the Pharisee leadership was at risk.  I agree Nicodemus shows courage by coming to Jesus but His religious beliefs and position kept him from receiving Jesus as the Messiah, the Son of God, and the things that Jesus shared about the kingdom of God