John 3:3-5
John 3:1-5
Now there was a man of the Pharisees, named Nicodemus, a ruler of the Jews. This man came to Jesus by night and said to him, "Rabbi, we know that you are a teacher come from God; for no one can do these signs that you do, unless God is with him." Jesus answered him, "I tell you the truth, unless one is born anew, he cannot see the kingdom of God." Nicodemus said to him, "How can a man be born when he is old? Can he enter a second time into his mother's womb and be born?" Jesus answered, "I tell you the truth, unless one is born of water and the Spirit, he cannot enter the kingdom of God.
It is an historical fact that certain sects of the Jews practiced baptism for gentile converts to Judaism during the first century (as they still do to this very day). So Nicodemus, who was a devout religious teacher of the day, would have certainly been familiar with the concept. This high-ranking religious teacher comes to Jesus at night and tries to engage Him in a conversation. Jesus could have chosen any topic to talk about.
Jesus “took the bull by the horns” and chose the topic of rebirth (v .3) through baptism (v.5).
Notice Jesus says unless one is born again he cannot see the kingdom of God. Nicodemus asks Jesus how can someone be reborn when they are old (He understands Jesus is NOT speaking of natural childbirth). Jesus describes it and tells him very plainly how—by being born “of water and the Spirit.” This is a perfect description of baptism! Jesus could not have given a more detailed and accurate explanation of baptism. Some may wonder why the Lord did not simply say “baptism”. But that would only be giving us the term and not the definition. Instead, Jesus wanted to be very specific, and in using the phrase “born of water and the Spirit” He gave us the very definition of baptism! Jesus tells Nicodemus one cannot enter the kingdom of heaven unless this happens. Look for the “water and the spirit” in Acts 2:38-41. According to Jesus Christ, who very clearly told Nicodemus the truth – unless a person is baptized, they cannot be saved.
Sadly, many “Christians” fight this truth to no end. But God has made it simple for us to follow His plan of salvation. God does not require us to build a rocketship to the moon, nor does He require us to do something difficult. What God requires us to be saved is to believe in His Son Jesus Christ, repent of our sins, and commit our life to Him, and get down into water and be baptized (immersed) for the purpose of having Him forgive our sins and be born again in this manner.
Some might be surprised to learn that Christian baptism actually has its roots in Judaism. During Jesus’ time, as today, Gentiles who wish to convert to Judaism immerse themselves in a water bath called a mikveh.* This procedure is called tevillah.
“The baptismal water (Mikveh) in rabbinic literature was referred to as the womb of the world, and as a convert came out of the water it was considered a new birth separating him from the pagan world. As the convert came out of these waters his status was changed and he was referred to as “a little child just born” or “a child of one day” (Yeb. 22a; 48b; 97b). We see the New Testament using similar Jewish terms as “born anew,” “new creation,” and “born from above.”
Regarding the practice of baptizing proselytes, Lightfoot, in Horae Hebraicae explains:
"As soon as he grows whole of the wound of circumcision, they bring him to Baptism, and being placed in the water they again instruct him in some weightier and in some lighter commands of the Law. Which being heard, he plunges himself and comes up, and, behold, he is an Israelite in all things."
It would have been abundantly obvious to Nicodemus, a teacher of all Israel, that Jesus was plainly speaking of tevillah, or mikvah, or what Christians nowadays call baptism. 
To learn more about the Jewish background of Christian baptism—click here.
There are some who teach being “born of water” means natural birth. But if that were the case, then the souls of babies that die in the womb or are miscarried or aborted (and never experience natural birth) would be excluded from the kingdom of heaven! From other places in Scripture, we know this simply not to be true because babies are innocent. If they die, they are saved and go to heaven. For a more in-depth look at the state of babies’ souls, click here.
Verse 6: Jesus is not talking about the rebirth of one’s flesh but the rebirth of man’s spirit—and the rebirth Jesus has in mind does not involve the flesh. It involves the Spirit. It wouldn’t matter if we could enter a hundred times into the womb again, it would still not yield the required result because flesh gives birth to flesh, and a rebirth of a man’s spirit is what is required.
*Glossary of Jewish terms
“Mikvah” – (f., pl. “Mikvaot”); a ritual pool of water, used for the purpose of attaining ritual purity. Immersion in a Mikvah is performed for the following main purposes: It is used in connection with Repentance, to remove the impurity of sin. It is also used in connection with Conversion, because the convert has taken upon himself or herself to adopt the lifestyle of the Jew, that is based on the recognition of G-d as King of the Universe and on the obligation to perform the commandments of the Torah.
For additional insight:
Even today, Jewish people still practice “tevillah” (or mikvah) which is what Christians would call "baptism". This is the same type of full water immersion in a “baptistry”—which Jewish people call a “mikvah”. It is a “baptism” where Christ is actually rejected or denied. Yes, Jewish people still practice this (and have been since at least Christ’s time and even before). It was around back then and is the background context in which the conversation between Nicodemus and Jesus took place. The ancient application of Mikvah is and was practiced for several different reasons, one of which is for conversion to Judaism. Click here and the global directory photo gallery to see an external, Jewish site for more information on Mikvah.
Mikveh, Tevillah, and Baptism -Examples of both ancient and modern Mikvehs
See also Mikvah and Conversion to Judaism encyclopedia entry.
Other Mikvah links (for reference only) to better understand context of John 3: