5 For why, my brethren? Who does not see what they do not? And no wonder; for they who are unwilling to return from that are just like the raven that was sent forth from the ark. For who does not see what they see not? They are unthankful even to the Holy Spirit Himself. See, the dove descended upon the Lord, upon the Lord when baptized: and thereupon was manifested that holy and real Trinity, which to us is one God. For the Lord went up out of the water, as we read in the Gospel: “And, lo, the heavens were opened unto Him, and He saw the Spirit descending like a dove, and it abode upon Him: and immediately a voice followed, You are my beloved Son, in whom I am well pleased.” The Trinity most manifestly appears: the Father in the voice, the Son in the man, the Spirit in the dove.
In this Trinity let us see, as we do see, whereunto the apostles were sent forth, and what it is wonderful those men do not see. Not indeed that they really do not see, but that they really shut their eyes to that which strikes them in the very face: that whereunto the disciples were sent forth in the name of the Father, and of the Son, and of the Holy Ghost, by Him of whom it is said, “This is He that baptizes:” it was said, in fact, to His ministers, by Him who has retained this authority to Himself.
Source: Tractates on the Gospel of John (New Advent)