Nicodemus knew that there was something different about Jesus, but he didn't want to ruin his rep with the other Pharisees by seeing him during the day. Some have said that coming at night might have meant that he just wanted to be able to talk with Jesus for a longer time, uninterupted. That is possible, but Jesus was not very popular with the Pharisees, he was taking their glory by not merely talking a good game and making the exterior look good, He impacted people's lives in love.
In order to enter the kingdom of God, be saved, we have to be born of water and born of the Spirit. There are a variety of possible interpretations of water, from ceremonial cleansing to natural birth, but the Spirit is clear. Unless you have the spiritual birth of the Holy Spirit dwelling within you, you are not born again or "begotten from above" as it may be translated and therefore will have no entrance into God's kingdom. This is God's work in your life.
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