Gregory the Great — as excerpted in the Catena Aurea on Luke 3:15-17

“(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.”