17 But how do we show that all nations belong to the “two or three metretæ apiece”? It was a matter of reckoning, in some measure, that he should say the same water-pots contained “two apiece,” which he had said contained “three apiece;” evidently in order to intimate to us a mystery therein. How are there “two metretæ apiece”? Circumcision and uncircumcision. Scripture mentions these two classes of people, and leaves out no kind of men, when it says, “Circumcision and uncircumcision;” in these two appellations you have all nations: they are the two metretæ apiece.
In these two walls, meeting from different quarters, “Christ became the corner-stone, in order to make peace in Himself.” Let us show also the “three metretæ apiece” in the case of these same all nations. Noah had three sons, through whom the human race was restored. Hence the Lord says, “The kingdom of heaven is like leaven, which a woman took and hid in three measures of meal, till the whole was leavened.” What is this woman, but the flesh of the Lord? What is the leaven, but the gospel?
What the three measures, but all nations, on account of the three sons of Noah? Therefore the “six water-pots containing two or three metretæ apiece” are six periods of time, containing the prophecy relating to all nations, whether as represented in two sorts of men, namely, Jews and Greeks, as the apostle often mentions them; or in three sorts, on account of the three sons of Noah. For the prophecy was represented as reaching unto all nations. And because of that reaching it is called a measure, even as the apostle says, “We have received a measure for reaching unto you.” For in preaching the gospel to the Gentiles, he says, “A measure for reaching unto you.”
Source: Tractates on the Gospel of John (New Advent)