Jesus said -
"Truly, truly, I say to you, unless one is born of water and 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." (John 3.5-6)
Here is a wonderful article published on line to help us understand all that Jesus was telling Nicodemus:
John and Becki Bennett Serving with NTM @ NTBI Waukesha 