7 And He goes on, “If I have told you earthly things, and you believe not; how shall you believe, if I tell you heavenly things?” What earthly things did He tell, brethren? “Unless a man be born again;” is that an earthly thing? “The Spirit blows where it lists, and you hear its voice, and know not whence it comes, or whither it goes;” is that earthly? For if He spoke it of the wind, as some have understood it, when they were asked what earthly thing the Lord meant, when He said, “If I told you earthly things, and you believe not; how shall you believe, if I tell you heavenly things?”— when, I say, it was asked of certain men what “earthly thing” the Lord meant, being in difficulty, they said, What He said, “The Spirit blows where it lists,” and “its voice you hear, and know not whence it comes, or whither it goes,” He said concerning the wind.
Now what did He name earthly? He was speaking of the spiritual birth; and going on, says, “So is every one that is born of the Spirit.” Then, brethren, which of us does not see, for example, the south wind going from south to north, or another wind coming from east to west? How, then, know we not whence it comes and whither it goes? What earthly thing, then, did He tell, which men did not believe? Was it that which He had said about raising the temple again? Surely, for He had received His body of the earth, and that earth taken of the earthly body He was preparing to raise up.
They did not believe Him as about to raise up earth. “If I told you earthly things,” says He, “and you believe not; how shall you believe if I tell you heavenly things?” That is, if you believe not that I can raise up the temple cast down by you, how shall you believe that men can be regenerated by the Spirit?
Source: Tractates on the Gospel of John (New Advent)