Jesus answered, "Most assuredly I tell you, unless one is born of water and spirit, he can't enter into the Kingdom of God!
When a person is truly baptized (and only believing, penitent, confessing persons can be TRULY baptized), as Christ commanded, God sends the Holy Spirit into his heart (the second element of the new birth); and, when viewed in connection with this divine fulfillment of the promise of the Holy Spirit, baptism is the new birth; but it is not a birth of water only, but a birth of "water and of the Spirit" as Jesus said.  On the other hand, when baptism is thought of as the water ceremony only, it is only part of the new birth, nevertheless a vital and necessary part of it.  It is proper to use baptism as a synecdoche for the new birth in its entirety; and thus Jesus himself used it in  Mark 16:16.