...we behold and see as it were in a divine spectacle exhibited to us, the notice of our God in Trinity, conveyed to us at the river Jordan. For when Jesus came and was baptized by John, the Lord by His servant (and this He did for an example of humility; for He shows that in this same humility is righteousness fulfilled, when as John said to Him, "I have need to be baptized by You, and You come to me?" He answered, "Suffer it to be so now, that all righteousness may be fulfilled" ), when He was baptized then, the heavens were opened, and the Holy Spirit came down upon Him in the form of a Dove: and then a Voice from on high followed, "This is My beloved Son, in whom I am well pleased." Here then we have the Trinity in a certain sort distinguished. The Father in the Voice — the Son in the Man — the Holy Spirit in the Dove.
Augustine, Sermon 2 on the New Testament.