Gregory the Great — as excerpted in the Catena Aurea on Luke 3:3-6

“(ubi sup.) Or John is said to preach the baptism of repentance for the remission of sins, because the baptism which was to take away sin, as he could not give, he preached; just as the Incarnate Word of the Father preceded the word of preaching, so the baptism of repentance, which was able to take away sin, was preceded by John’s baptism, which could not take away sin.”