2 Of this pool, which was surrounded with five porches, in which lay a great multitude of sick folk, I remember that I have very often treated; and most of you will with me recollect what I am about to say, rather than gain the knowledge of it for the first time. But it is by no means unprofitable to go back upon matters already known, that both they who know not may be instructed, and they who do know may be confirmed. Therefore, as being already known, these things must be touched upon briefly, not leisurely inculcated.
That pool and that water seem to me to have signified the Jewish people. For that peoples are signified under the name of waters the Apocalypse of John clearly indicates to us, where, after he had been shown many waters, and he had asked what they were, was answered that they were peoples. That water, then— namely, that people— was shut in by the five books of Moses, as by five porches. But those books brought forth the sick, not healed them. For the law convicted, not acquitted sinners.
Accordingly the letter, without grace, made men guilty, whom on confessing grace delivered. For this is what the apostle says: “For if a law had been given which could have given life, verily righteousness should have been by the law.” Why, then, was the law given? He goes on to say, “But the Scripture has concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.” What more evident? Have not these words expounded to us both the five porches, and also the multitude of sick folk?
The five porches are the law. Why did not the five porches heal the sick folk? Because, “if there had been a law given which could have given life, verily righteousness should have been by the law.” Why, then, did the porches contain those whom they did not heal? Because “the Scripture has concluded all under sin, that the promise by faith of Jesus Christ might be given to them that believe.”
Source: Tractates on the Gospel of John (New Advent)