The Venerable Bede writes:
The angel designates the one who descended invisibly into the pool and moved the water to provide the power of healing. Clothed in flesh, Christ descended into the water as the Angel of great counsel [Is 9:6], that is, as a herald of the Father's will to the Jewish people... The movement of the water suggests the Lord's Passion, which occurred by the nation of the Jews being moved and stirred up. And because through His Passion those who believed were redeemed from the curse of the law, it is as if they were healed as they descended into the troubled water of the pool... It is good that the one who first went down after the movement of the waster was healed of whatever illness had him in its grip, for there is one Lord, one faith, one baptism [Eph 4:5]; and a person, who in catholic unity is imbued with the mysteries of Christ, is healed from whatever illness, caused by his sins, holds him fast. Whoever is out of harmony with that unity is not capable of securing salvation, since it is from One.