Ambrose of Milan
Catena Aurea · Lukasevangelium 3
Now when all the people were baptized, it came to pass, that Jesus also being baptized, and praying, the heaven was opened, And the Holy Ghost descended in a bodily shape like a dove upon him, and a voice came from heaven, which said, Thou art my beloved Son; in thee I am well pleased.
In a matter which has been related by others, Luke has rightly given us only a summary, and has left more to be understood than expressed in the fact, that our Lord was baptized by John. As it is said, Now when all were baptized, it came to pass. Our Lord was baptized not that He might be cleansed by the waters but to cleanse them, that being purified by the flesh of Christ who knew no sin, they might possess the power of baptism.