The root and trunk of the born-again doctrine is found within John 3. Matthew, Mark, and Luke do not speak directly of it at all, though without directly naming it, they supply supporting information. It is not until the epistles of Paul, Peter, and especially John that main branches of this doctrine make appearances. Thus, as we begin, it is helpful for us to perceive the wide treatment of figures John uses to prepare us for how he uses them to support the various elements of this important, foundational doctrine.
He begins using symbolism immediately in John 1, identifying Jesus as the Word, the central Figure in God's spiritual work in men's behalf. He continues, speaking of light, darkness, baptism, the Lamb of God, and the Temple, among others, before the reader arrives at John 3.
The imagery regarding the Temple (John 2:18-22) is especially interesting because it immediately precedes Jesus' teaching on born again in John 3. The Jews listening to Jesus immediately reject what He teaches based on what He says being a physical impossibility. Indeed, it is physically impossible, but note that this is the same reason Nicodemus rejects Jesus' teaching on born again. Similarly, in John 4:7-15, the woman at the well immediately jumps to the conclusion that Jesus speaks of natural water, and in John 4:31-38, even Jesus' disciples fail to grasp the spiritual significance of food.
In John 6:32-63, those who listen to His manna discourse follow the same pattern. In fact, His "eat My flesh and drink My blood" imagery so offends many of His disciples that they stop following Him! This consistent failure to grasp the meaning of His imagery continues through the entire book. If, in studying John 3, we follow the same pattern of misunderstanding His spiritual imagery, like Nicodemus, we will also misunderstand being born again.
We must recognize that this spiritualizing continues in John 3. In fact, for the children of God, it not only continues, but it also increases exponentially in terms of its importance to their spiritual lives! It is an unvarnished truth that only those who are born again will see and enter the Kingdom of God (John 3:5). Jesus is teaching that, besides one's biological birth, one must also experience a supernatural, spiritual birth. Just as surely as a Christian is not merely biologically begotten but born, there is no such thing as a non-born-again Christian.