The Gospel of John, the latest of the canonical gospels, written at least 50 years after the crucifixion and 15 years after the Gospel of Mark, contains many events and passages not found in the other (so called “synoptic”) gospels. It is the only gospel in which Jesus openly calls himself the son of God. Nevertheless, many Christians consider it to be the most spiritually true of the gospels. While its author was actually anonymous (claiming only to be Jesus’s favorite disciple), for simplicity, I will follow tradition and call him John. [See the comments for a discussion on whether the author was actually John son of Zebedee; I personally make no claims to know.]
The following story is unique to the Gospel of John. A Pharisee named Nicodemus, one of the leaders of the Jews, came to Jesus one night:
“Rabbi,” he said, “I know that you are a teacher sent by God. No man can perform the miracles that you do if God is not with him.”
“Truly, I tell you,” said Jesus, “that unless you are born again, you will not see the kingdom of heaven.”
“How can a man be born when he is old?” asked Nicodemus. “Can he reenter his mother’s womb, and be born a second time?”
“Unless you are born from water and the holy Spirit, you cannot enter the kingdom of heaven,” said Jesus. “Those born from the flesh are flesh, only those who are reborn from the Spirit are spirit. Do not be surprised that I say you must be born again. The wind blows where it wants, and you hear the sound of it, but cannot tell where it comes from, and where it goes. So too, the Spirit comes and goes as it will, touching every one who is born of it.”
“How can this be?” asked Nicodemus.
“Do you, a master of Israel, not know these things?” replied Jesus. “When I speak of what I know, and testify to what I have seen, the people do not believe me. If I tell them of material things and them do not believe me, how will they believe if I tell them of spiritual things? No man can ascend to heaven but the one that came down from heaven, the Son of Man. As Moses lifted the plague of serpents in the wilderness, so must the Son of Man lift our affliction. Whoever believes in him shall not die, but shall instead have eternal life. God so loves the world that he will give us his only begotten Son. He will not send his Son into the world to condemn it, but so that through him, it can be saved. He who believes will not be condemned, but he that does not believe is already condemned, because he has not believed in the name of the only begotten Son of God. This condemnation is because light has come into the world, but some men love darkness rather than light, because their deeds are evil. Every one that does evil hates the light, and will not come into it, lest his deeds should be reproved. But he that acts truly comes into the light to make his deeds manifest, that they are done under the eyes of heaven.”
John mixes the message of the coming kingdom of heaven, ushered in by the cosmic judge, the Son of Man, who will descend from heaven and make the righteous immortal by the power of the holy Spirit, with the message of redemption by the Son of God. He does this by equating the Son of Man, the apocalyptic figure from the book of Daniel, with the Son of God, who he believed Jesus to be. He also mixes the idea that to enter the kingdom of heaven, one must be baptized and must act truly with the message that one must have faith in the Son of God.