"Except a man be born again, he cannot see the kingdom of God." (John 3:3)
To be a Christian means to be born again... and that means to accept Jesus Christ as your savior, so-called because the spirit of God enters you and you become a "new creature" (II Corinthians 5). When you are born as an infant into this world, you are born of flesh, with the spirit of man. But when you pray to God, asking him to forgive your sins in Jesus' name, asking Jesus to come into your heart and save you from the death that you deserve, then you are born again, of the spirit of God. That is what it means to be a Christian. To be born again, saved from sin and death.
It's not enough to
"believe in God"...
even the devil believes in God.
Accepting Jesus Christ is an act of faith alone. Until you accept him as your savior, you may not understand or believe all the things that God says.
Why do ye not understand my speech? even because ye cannot hear my word. Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it. And because I tell you the truth, ye believe me not. (John 8: 43-45)
Being born again is simple, but you must make the choice. And once you do, you aren't under any magical spell that makes you a Christian. You choose every day to follow Jesus rather than following the things of this world. Here are a few verses that explain these things.
We have all sinned against God, and death is the penalty for sin...
Romans 3: 23
For all have sinned, and come short of the glory of God...
Romans 3: 10
As it is written, There is none righteous, no, not one:
The wages of sin is death...
John 8: 24
I said therefore unto you, that ye shall die in your sins: for if ye believe not that I am he, ye shall die in your sins.
Matthew 9: 13
I am not come to call the righteous, but sinners to repentance.
...but Jesus Christ paid our penalty. He died for us, so that if we repent and believe in him we can be freed from sin and death, and reunited with God:
Romans 10: 9-13
If thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. For the scripture saith, Whosoever believeth on him shall not be ashamed. For there is no difference between the Jew and the Greek: for the same Lord over all is rich unto all that call upon him. For whosoever shall call upon the name of the Lord shall be saved.
John 3: 3-7
Jesus answered... Except a man be born again, he cannot see the kingdom of God. Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of 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. Marvel not that I said unto thee, Ye must be born again.
Acts 4: 10,12
Jesus Christ of Nazareth, whom ye crucified, whom God raised from the dead... Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.
Ephesians 2: 4-9
God, who is rich in mercy, for his great love wherewith he loved us, Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;) And hath raised us up together, and made us sit together in heavenly places in Christ Jesus: That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus. For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Not of works, lest any man should boast.
John 1: 12
But as many as received him, to them gave he power to become the sons of God, even to them that believe on his name: