3 What was done, then, that they who could not be healed in the porches might be healed in that water after being troubled? For on a sudden the water was seen troubled, and that by which it was troubled was not seen. You may believe that this was wont to be done by angelic virtue, yet not without some mystery being implied. After the water was troubled, the one who was able cast himself in, and he alone was healed: whoever went in after that one, did so in vain. What, then, is meant by this, unless it be that there came one, even Christ, to the Jewish people; and by doing great things, by teaching profitable things, troubled sinners, troubled the water by His presence, and roused it towards His own death? But He was hidden that troubled. For had they known Him, they would never have crucified the Lord of glory. Wherefore, to go down into the troubled water means to believe in the Lord's death. There only one was healed, signifying unity: whoever came thereafter was not healed, because whoever shall be outside unity cannot be healed.
Source: Tractates on the Gospel of John (New Advent)