"Do not be amazed that I said to you, ‘You must be born again.’"
Several days ago I had a conversation with a man and woman who came into my shop. When they saw some of the faith-related merchandise I carry they asked, "Are you a Christian?" "Yes," I answered. "A born-again Christian?" they asked. I answered "yes." After they left I thought about our exchange. In actuality there is no such thing as a Christian who is not born again. If you're not born again, you're not truly a Christian. I didn't make this up. Jesus said it Himself: “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. Do not be amazed that I said to you, ‘You must be born again.’"
When a Christian is born again in the spirit, his old self dies and his new self lives. It's a spiritual rebirth. "Therefore we have been buried with Him through baptism into death, so that as Christ was raised from the dead through the glory of the Father, so we too might walk in newness of life. For if we have become united with Him in the likeness of His death, certainly we shall also be in the likeness of His resurrection, knowing this, that our old self was crucified with Him, in order that our body of sin might be done away with, so that we would no longer be slaves to sin; for he who has died is freed from sin. Now if we have died with Christ, we believe that we shall also live with Him, knowing that Christ, having been raised from the dead, is never to die again; death no longer is master over Him. For the death that He died, He died to sin once for all; but the life that He lives, He lives to God. Even so consider yourselves to be dead to sin, but alive to God in Christ Jesus." (Romans 6:4-11)
Jesus said, "Don't be amazed that I said to you 'You must be born again.'" He said it. I believe it. That settles it. We must be born again to be a Christian.