What is the Christian doctrine of regeneration?

from the old testament

• The Old Testament doesn’t use the word “regeneration” the way the New Testament does, but it does teach the principle that God can transform and renew a person’s heart. Regeneration refers to the spiritual rebirth God gives, making a person alive to Him and able to obey Him. In Ezekiel 36:26 God tells His people, “And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh.”
• Psalm 51:10 recognizes that personal transformation is only possible by God’s work. After committing adultery with Bathsheba, having her husband killed, and being confronted by God about through the prophet Nation, David pleads with God, “Create in me a clean heart, O God.”
• Spiritual regeneration brings life and blessing (Isaiah 44:3).

from the new testament

• Regeneration is a theological term from the King James Version of the Bible. Titus 3:5 says, "he saved us, not because of works done by us in righteousness, but according to his own mercy, by the washing of regeneration and renewal of the Holy Spirit." Modern translations use words such as rebirth to communicate the same idea.
• The concept of being reborn is of great importance in the Bible. In John 3 a Jewish religious teacher named Nicodemus visited Jesus at night to learn more about Him. Jesus told him, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God" (John 3:3). When Nicodemus questioned the idea of rebirth, Jesus replied, "Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter the kingdom of God" (John 3:5). To be "born again" or "regenerated" is to start a new spiritual life, one that is from God rather than from a human parent.
• Other passages affirm regeneration. Ephesians 2:1-2 notes that unbelievers are "dead in the trespasses and sins" prior to knowing Christ. God's salvation brings them from death to life (Ephesians 2:5-6).
• Being born again brings radical change: "Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come." (2 Corinthians 5:17).
• John 3:16 explains why God offers this new, regenerated life: "For God so loved the world, that he gave his only Son, that whoever believes in him should not perish but have eternal life." God's love led Him to give His Son on our behalf so we could have eternal life.

implications for today

How does regeneration take place? Both John 3 and Ephesians 2 indicate that this change is the result of faith in Christ. Just as an infant does nothing to be born into this world, no work a person can do grants them spiritual rebirth. God gives new life by His grace to those who trust in Him. Ephesians 2:8-9 says, "For by grace you have been saved through faith. And this is not your own doing; it is the gift of God, not a result of works, so that no one may boast."

Of course, after someone is born, it's time to grow. We would rightly be concerned for an infant who remained at infant size with infant abilities for years on end. In the same way, a person who has been regenerated by the power of the Holy Spirit should grow spiritually. Growth naturally follows birth. "Like newborn infants, long for the pure spiritual milk, that by it you may grow up into salvation" (1 Peter 2:2; see also 2 Peter 3:18).

Spiritual rebirth is a one-time event. There is no biblical support for a person to be born again multiple times. Once a person truly comes to faith in Christ, a new life begins, continuing from that point through eternity future with the Lord.