20 I do not reject John, but rather I believe John. In what do I believe John? In that which he learned through the dove? What did he learn through the dove? “This is He which baptizes with the Holy Ghost.” Now therefore, brethren, hold this fast and impress it upon your hearts; for if I would more fully explain today, Wherefore through the dove? time fails. For I have, I think, to some extent made plain to you, holy brethren, that a matter which had to be learned was instilled into John by means of the dove, a matter with regard to Christ which John did not know, although he already knew Christ; but why it behooved this matter to be pointed out by means of the dove, I would say, were it possible to say it briefly: but because it would take long to say, and I am unwilling to burden you, since I have been helped by your prayers to perform my promise; with the renewed help of your pious attention and good wishes, it will likewise become clear to you, wherefore John with regard to that matter which he learned regarding the Lord, namely, that it is “He which baptizes with the Holy Ghost,” and that to none of His servants had he transferred the power of baptizing— why this it became him not to learn except through the dove.
Source: Tractates on the Gospel of John (New Advent)