Simple. Read the letter.
*1 John 5:1-12*
Everyone who believes that Jesus is the Christ has been born of God, and everyone who loves the Father loves whoever has been born of him. By this we know that we love the children of God, when we love God and obey his commandments. For this is the love of God, that we keep his commandments. And his commandments are not burdensome. For everyone who has been born of God overcomes the world. And this is the victory that has overcome the world—our faith. Who is it that overcomes the world except the one who believes that Jesus is the Son of God? This is he who came by water and blood—Jesus Christ; not by the water only but by the water and the blood. And the Spirit is the one who testifies, because the Spirit is the truth. For there are three that testify: the Spirit and the water and the blood; and these three agree. If we receive the testimony of men, the testimony of God is greater, for this is the testimony of God that he has borne concerning his Son. Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son. And this is the testimony, that God gave us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life.
Now connect it to John 3:3-15.
Jesus answered 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? Can he enter a second time into his mother’s womb and be born?” Jesus answered, “Truly, truly, I say to you, unless one is born of water and the Spirit, he cannot enter 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 marvel that I said to you, ‘You must be born again.’ The wind blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.” Nicodemus said to him, “How can these things be?” Jesus answered him, “Are you the teacher of Israel and yet you do not understand these things? Truly, truly, I say to you, we speak of what we know, and bear witness to what we have seen, but youdo not receive our testimony. If I have told you earthly things and you do not believe, how can you believe if I tell you heavenly things? No one has ascended into heaven except he who descended from heaven, the Son of Man. And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, that whoever believes in him may have eternal life.
Do you see the fact that God must cause you to be born again before you can believe?
Paul says the same thing.
*Ephesians 2:1-9*
And you were dead in the trespasses and sins in which you once walked, following the course of this world, following the prince of the power of the air, the spirit that is now at work in the sons of disobedience— among whom we all once lived in the passions of our flesh, carrying out the desires of the body and the mind, and were by nature children of wrath, like the rest of mankind. But God, being rich in mercy, because of the great love with which he loved us, even when we were dead in our trespasses, made us alive together with Christ—by grace you have been saved— and raised us up with him and seated us with him in the heavenly places in Christ Jesus, so that in the coming ages he might show the immeasurable riches of his grace in kindness toward us in Christ Jesus. 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.