Jesus told Nicodemus, “You're a well-known teacher of Israel. Can't you understand this?- John 3:10
Nicodemus was told that he had to be born again and being the practical man that he was he couldn’t understand this concept. Jesus even explained that there was physical birth and there was spiritual birth, yet Nicodemus was confused and doubting. Jesus explained that the wind came and went without our ever seeing it yet we knew it was blowing. Yet Nicodemus still questioned. I think Nicodemus was trying to use his intellect to understand spiritual matters.
He had to figure it out for himself. To a certain extent this is a good thing. Belief in Jesus Christ is not mindless. Nicodemus, Paul, Augustine, Tertullian and many other early church leaders were very intelligent and educated men. Many of our pastors and teachers today are above and beyond in intelligence. But you cannot fully intellectualize the gospel. The Christian faith is called faith for a reason. You can come to logical conclusions and see the advantage of believing in Jesus, but you must still believe. And belief takes faith. And faith comes from God. So how do you believe if you don’t have faith and only God can provide that faith? The answer is as simple as salvation itself – you ask for it. Ask God and He will provide all of your needs, including the faith needed to receive salvation.