Simple....
John 3:3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God. John 3:7 Marvel not that I said unto thee, Ye must be born again. 1 Peter 1:23 Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.
Its about first being born in the flesh and then being born in the Spirit