Jesus is very clear that being born again of the Spirit (John 3:5) is the only way to see/enter the kingdom of God. A few chapters later in the Book of John, Jesus confirms the importance of the Spirit when talking to his disciples:
It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. (John 6:63)
For a study on the spirit and the flesh, see: The Spirit Quickeneth but the Flesh Profiteth Nothing.
John 3 Audio
1 There was a man of the Pharisees, named Nicodemus, a ruler of the Jews:
2 The same came to Jesus by night, and said unto him, Rabbi, we know that thou art a teacher come from God: for no man can do these miracles that thou doest, except God be with him.
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.
4 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?
5 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.
6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
7 Marvel not that I said unto thee, Ye must be born again.
8 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.