10 “The Jews said unto Him, What sign do you show unto us, seeing that you do these things?” And the Lord answered, “Destroy this temple, and in three days I will raise it up. Then said the Jews, Forty and six years was this temple in building, and do you say, In three days I will rear it up?” Flesh they were, fleshly things they minded; but He was speaking spiritually. But who could understand of what temple He spoke? But yet we have not far to seek; He has discovered it to us through the evangelist, he has told us of what temple He said it. “But He spoke,” says the evangelist, “of the temple of His body.” And it is manifest that, being slain, the Lord did rise again after three days. This is known to us all now: and if from the Jews it is concealed, it is because they stand without; yet to us it is open, because we know in whom we believe. The destroying and rearing again of that temple, we are about to celebrate in its yearly solemnity: for which we exhort you to prepare yourselves, such of you as are catechumens that you may receive grace; even now is the time, even now let that be purposed which may then come to the birth. Now, that thing we know.
Source: Tractates on the Gospel of John (New Advent)