Gregory the Great
Catena Aurea · Lukasevangelium 3
And as the people were in expectation, and all men mused in their hearts of John, whether he were the Christ, or not; John answered, saying unto them all, I indeed baptize you with water; but one mightier than I cometh, the latchet of whose shoes I am not worthy to unloose: he shall baptize you with the Holy Ghost and with fire. Whose fan is in his hand, and he will throughly purge his floor, and will gather the wheat into his garner; but the chaff he will burn with fire unquenchable.
(Hom. 7. in Evan.) But John denounces himself as unworthy to loose the latchet of Christ’s shoes: as if he openly said, I am not able to disclose the footsteps of my Redeemer, who do not presume unworthily to take unto myself the name of bridegroom, for it was an ancient custom thata when a man refused to take to wife her whom he ought, whoever should come to her betrothed by right of kin, was to loose his shoe. Or because shoes are made from the skins of dead animals, our Lord being made flesh appeared as it were with shoes, as taking upon Himself the carcase of our corruption. The latchet of the shoe is the connexion of the mystery. John therefore can not loose the latchet of the shoe, because neither is he able to fathom the mystery of the Incarnation, though he acknowledged it by the Spirit of prophecy.