7 Mark, brethren; before our Lord Jesus Christ came to His baptism (for it was after the baptism that the dove descended, whereby John recognized something that was peculiar to Him, since he was told, “Upon whom you shall see the Spirit descending like a dove, and remaining on Him, the same is He that baptizes with the Holy Ghost”), John knew that He it was that baptizes with the Holy Ghost; but that it should be with this peculiarity, that the authority should not pass from Him to another, notwithstanding He confers it, this is what he learned there.
And whence do we prove that John did already know that the Lord was to baptize with the Holy Ghost; so that what he must be understood to have learned by the dove is, that the Lord was to baptize with the Holy Ghost in such wise that the authority should not pass from Him to any other man? Whence do we prove this? The dove descended after the Lord was baptized; but before the Lord came to be baptized by John in the Jordan, we have said that John knew Him, on the evidence of those words, in which he says, “Comest Thou to me to be baptized?
I have need to be baptized by You.” Well, he did know Him to be the Lord, knew Him to be the Son of God; how do we prove that he knew already that the same was He who should baptize with the Holy Ghost? Before He came to the river, while many people were running together to John to be baptized, he says to them, “I indeed baptize you with water; but He that comes after me is greater than I, the latchet of whose shoes I am not worthy to loose; the same shall baptize you with the Holy Ghost, and with fire.” Already he knew this also.
What then did he learn from the dove, that he may not afterwards be found a liar (which God forbid we should think), if it be not this, that there was to be a certain peculiarity in Christ, such that, although many ministers, be they righteous or unrighteous, should baptize, the virtue of baptism would be attributed to Him alone on whom the dove descended, and of whom it was said, “This is He that baptizes with the Holy Ghost”? Peter may baptize, but this is He that baptizes; Paul may baptize, yet this is He that baptizes; Judas may baptize, still this is He that baptizes.
Source: Tractates on the Gospel of John (New Advent)