Theophylact of Ohrid

Many thought that the water received a certain divine power from the mere fact that the entrails of the sacrifices were washed in it, and that therefore the Angel also descended upon this water as upon a chosen one and worked miracles. It seems that Divine Providence pre-ordained the miracle in this pool in order to lead the Jews from afar to faith in Christ. …

Commentary on John ↗

Augustine of Hippo

So then Christ came to the Jewish people, and by means of mighty works, and profitable lessons, troubled the sinners, i. e. the water, and the stirring continued till He brought on His own passion. But He troubled the water, unknown to the world. For had they known Him, they would not have crucified the Lord of glory. (1 Cor. 11) But the troubling of the water came on all at once, and it was not seen who troubled it. Again, to go down into the troubled water, is to believe humbly on our Lord's passion. …

Catena Aurea by Aquinas ↗

Ambrose of Milan

What did the angel declare in this type but the descent of the Holy Spirit, which was to come to pass in our day and should consecrate the waters when invoked by the prayers of the priest? That angel, then, was a herald of the Holy Spirit, inasmuch as by means of the grace of the Spirit medicine was to be applied to our infirmities of soul and mind. The Spirit, then, has the same ministers as God the Father and Christ. …

On the Holy Spirit 1.8.88 ↗

Thomas Aquinas

Finally, the power of the pool is described, for it healed all physical illnesses in virtue of an angel who came to it; so he says, From time to time an angel of the Lord used to come down into the pool. In certain ways, the power of this pool is like that of Baptism. It is like it, first, in the fact that its power was unperceived: for the power of the water in this pool did not come from its very nature, otherwise it would have healed at all times; its power was unseen, being from an angel. …

Commentary on John ↗