The Answer is right from the mouth of Jesus himself.
John 3:3
3 Jesus answered and said to him, “Most assuredly, I say to you, unless one is born -again, he cannot see the kingdom of God.”
John3:16-17
16 For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. 17 For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.
many stops at that verses but Jesus went on to say :
:18
18 “He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God.