5 When these words of the Gospel are understood, and they are certainly clear, all the mysteries which are latent in this miracle of the Lord will be laid open. Observe what He says, that it behooved the things to be fulfilled in Christ that were written of Him. Where were they written? “In the law,” says He, “and in the prophets, and in the Psalms.” He omitted no part of the Old Scriptures. These were water; and hence the disciples were called irrational by the Lord, because as yet they tasted to them as water, not as wine.
And how did He make of the water wine? When He opened their understanding, and expounded to them the Scriptures, beginning from Moses, through all the prophets; with which being now inebriated, they said, “Did not our hearts burn within us in the way, when He opened to us the Scriptures?” For they understood Christ in those books in which they knew Him not before. Thus our Lord Jesus Christ changed the water into wine, and that has now taste which before had not, that now inebriates which before did not.
For if He had commanded the water to be poured out of the water-pots, and so Himself had put in the wine from the secret repositories of the creature, whence He made bread when He satisfied so many thousands; for five loaves were not in themselves sufficient to satisfy five thousand men, nor even to fill twelve baskets, but the omnipotence of the Lord was, as it were, a fountain of bread; so likewise He might, on the water being poured out, have poured in wine: but had He done this, He would appear to have rejected the Old Scriptures.
When, however, He turns the water itself into wine, He shows us that the Old Scripture also is from Himself, for at His own command were the water-pots filled. It is from the Lord, indeed, that the Old Scripture also is; but it has no taste unless Christ is understood therein.
Source: Tractates on the Gospel of John (New Advent)