9 Already, then, John knew this, and he knew the Lord. What then did the dove teach? What did He desire to teach by means of the dove— that is, by means of the Holy Spirit thus coming to teach who had sent him to whom He said, “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He”? Who is this He? The Lord? I know. But did you already know this, that the same Lord having the power to baptize, was not to give that power to any servant, but to retain it to Himself, so that all who were baptized by the ministration of the servant, should not impute their baptism to the servant, but to the Lord?
Did you already know this? I did not know this: so what did He say to me? “Upon whom you shall see the Spirit descending as a dove, and abiding upon Him, the same is He who baptizes with the Holy Ghost.” He does not say, “He is the Lord;” He does not say, “He is the Christ;” He does not say, “He is God;” He does not say, “He is Jesus;” He does not say, “He is the One who was born of the Virgin Mary, after you, before you.” This He does not say, for this John did already know. But what did he not know?
That this great authority of baptism the Lord Himself was to have, and to retain to Himself, whether present in the earth or absent in body in the heaven, and present in majesty; lest Paul should say, my baptism; lest Peter should say, my baptism. Therefore see, give heed to the words of the apostles. None of the apostles said, my baptism. Although there was one gospel of all, yet you find that they said, my gospel: you do not find that they say, my baptism.
Source: Tractates on the Gospel of John (New Advent)