The Christian concept of being born of water comes from the Bible’s passage in the gospel of John, chapter 3:
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 and said to him, “Most assuredly, I say to you, unless one is born again, 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, “Most assuredly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. Do not marvel that I said to you, ‘You must be born again.’ The wind blows where it wishes, and you hear the sound of it, but cannot tell where it comes from and where it goes. So is everyone who is born of the Spirit.” ~ John 3:1-8
To understand what it means to be born of water, as Jesus told Nicodemus, it is necessary to find out what the Bible has to say about water, in relation to Christians.
Paul described the water as, “the word” which cleanses the church; which would be, Scripture.
Husbands, love your wives, just as Christ also loved the church and gave Himself for her, that He might sanctify and cleanse her with the washing of water by the word, that He might present her to Himself a glorious church, not having spot or wrinkle or any such thing, but that she should be holy and without blemish. ~ Ephesians 5:25-27
Jesus also mentioned this in the gospel of John:
You are already clean because of the word which I have spoken to you. ~ John 15:3
Paul, in Titus 3:3-6, also described the Holy Spirit as having the same effect when sinners are regenerated. He also described regeneration as being “made alive”.
And you He made alive, who were dead in trespasses and sins, in which you once walked according to the course of this world, according to the prince of the power of the air, the spirit who now works in the sons of disobedience, among whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others. But God, who is rich in mercy, because of His great love with which He loved us, even when we were dead in trespasses, made us alive together with Christ (by grace you have been saved) ~ Ephesians 2:1-5
Therefore, the word of God, and the Holy Spirit, work together to regenerate sinners into children of God, by cleansing the hearts from sin, and making them new creations. That is why the Apostle John described Jesus as coming by water and by blood.
This is He who came by water and blood — Jesus Christ; not only by water, but by water and blood… ~ 1 John 5:6
Jesus came preaching the gospel of peace with God, and by shedding his blood in order to establish this peace (Ephesians 2:14-17). When those whom God draws to Christ hear the message of the gospel, they are regenerated. As Paul stated:
…if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new. ~ 2 Corinthians 5:17
And John stated:
…as many as received Him, to them He gave the right to become children of God, to those who believe in His name: who were born, not of blood, nor of the will of the flesh, nor of the will of man, but of God. ~ John 1:12-13
And the Apostle James stated:
Of His own will He brought us forth by the word of truth, that we might be a kind of firstfruits of His creatures. ~ James 1:18
As the Apostle Peter stated:
Since you have purified your souls in obeying the truth through the Spirit in sincere love of the brethren, love one another fervently with a pure heart, having been born again, not of corruptible seed but incorruptible, through the word of God which lives and abides forever, because “All flesh is as grass, And all the glory of man as the flower of the grass. The grass withers, And its flower falls away, But the word of the LORD endures forever.” Now this is the word which by the gospel was preached to you. ~ 1 Peter 1:22-25
This is the doctrine of regeneration. It is fulfillment of the promised made by God through Jeremiah.
Behold, the days are coming, says the LORD, when I will make a new covenant with the house of Israel and with the house of Judah — not according to the covenant that I made with their fathers in the day that I took them by the hand to lead them out of the land of Egypt, My covenant which they broke, though I was a husband to them, says the LORD. But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. No more shall every man teach his neighbor, and every man his brother, saying, ‘Know the LORD,’ for they all shall know Me, from the least of them to the greatest of them, says the LORD. For I will forgive their iniquity, and their sin I will remember no more. ~ Jeremiah 31:31-34
This promise is fulfilled by the preaching of the word (Romans 10:14-17), and it is performed by the living word of God, and by the power of the Holy Spirit within the hearts and minds of sinners.