...What is the connection between Jesus's birth and our new birth? What is the relationship Jesus's incarnation and our regeneration? Could not God have simply caused sinners to be born again and then finally conformed them to his own character in heaven, without sending his Son into the world? Did there need to be an incarnation of the Son of God and a perfect life of obedience and a death on the cross? The answer is: The new birth and all of its effects, including faith and justification and purification and final conformity to Christ in heaven, would be impossible without the incarnation and life and death of Jesus—without Christmas. Let's get a glimpse of this from 1 John 1. And may your love for Christ and his coming increase because of this glimpse.
First, consider that the aim of the new birth is to enable us to believe in the incarnate Jesus Christ. If there were no Jesus Christ to believe in, then the new birth would not happen. Look at 1 John 5:1: "Everyone who believes that Jesus is the Christ [that is everyone who believes that this incarnate Jewish man from Nazareth is the promised divine Messiah] has been born of God." That means that the Holy Spirit causes people to be born again with a view to creating faith in the incarnate God-man, Jesus Christ (see 1 John 4:2-3). That's the aim of the new birth. And so faith in Jesus Christ is the first evidence that it has happened. "Everyone who believes that Jesus is the Christ has been born of God." Faith is the sign that the new birth has happened.
But that's not the only reason the incarnation is necessary for the new birth—not just because the aim of the new birth is faith in Jesus Christ. The incarnation of the Son of God is necessary because the life we have through the new birth is life in union with the incarnate Christ. Jesus said, "I am the living bread that came down from heaven. If anyone eats of this bread, he will live forever. And the bread that I will give for the life of the world is my flesh" (John 6:51). That life that we have in union with Christ is the life that Jesus obtained for us by the life he lived and the death he died in the flesh.
Excerpted from "The Reason the Son of God Appeared Was to Destroy the Works of the Devil" by John Piper. © Desiring God. Website: desiringGod.org.