John 3: 1-6 Being born of the spirit.

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 have come from Godas a teacher; for no one can do these⁾signs that You do unless God is with him.” Jesus answered and said to him,” Truly, truly, 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? He cannot enter a second time into his mother’s womb and be born, can he?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God. That which is born of the flesh is flesh, and that which is born of the Spirit is spirit.

Whenever you hear a Christian speak of being "born again," they speak this way because it is the teaching of Jesus. Their are other expressions that mean the same thing. For example, if a person is "regenerated," he is born again. The same is true for "transformation." In each of these cases, it is a birth of the inner man and not the physical person. It is a new ability to discern the teaching of God that a person is not born with as an infant. This regeneration means simply that the Christian is now indwelt with the Holy Spirit of God and this is why it is required to inherit eternal life.