Mark 1:7

(Vict. Ant. e Cat. in Marc.) But lest he should be thought to say this by way of comparing himself to Christ, he subjoins, Of whom I am not worthy, &c. It is not however the same thing to loose the shoe-latchet, which Mark here says, and to carry his shoes, which Matthew says. And indeed the Evangelists following the order of the narrative, and not able to err in any thing, say that John spoke each of these sayings in a different sense. But commentators on this passage have expounded each in a different way. For he means by the latchet, the tie of the shoe. (non occ.). He says this therefore to extol the excellence of the power of Christ, and the greatness of His divinity; as if he said, Not even in the station of his servant am I worthy to be reckoned. For it is a great thing to contemplate, as it were stooping down, those things which belong to the body of Christ, and to see from below the image of things above, and to untie each of those mysteries, about the Incarnation of Christ, which cannot be unravelled.

(Hom. in Evan. vii.) Shoes also are made from the skins of dead animals. The Lord, therefore, coming incarnate, appeared us it were with shoes on His feet, for He assumed in His divinity the dead skins of our corruption. Or else; it was a custom among the ancients, that if a man refused to take as his wife the woman whom he ought to take, he who offered himself as her husband by right of kindred took off that man's shoe. Rightly then does he proclaim himself unworthy to loose his shoe-latchet, as if he said openly, I cannot make bare the feet of the Redeemer, for I usurp not the name of the Bridegroom, a thing which is above my deserts.

(ubi sup.) Thus then John proclaims the Lord not yet as God, or the Son of God, but only as a man mightier than himself. For his ignorant hearers were not yet capable of receiving the hidden things of so great a Sacrament, that the eternal Son of God, having taken upon Him the nature of man, had been lately born into the world of a virgin; but gradually by the acknowledgment of His glorified lowliness, they were to be introduced to the belief of His Divine Eternity. To these words, however, he subjoins, as if covertly declaring that he was the true God, I baptize you with water, but he shall baptize you with the Holy Ghost. For who can doubt, that none other but God can give the grace of the Holy Ghost.

Some persons also understand it thus; all who came to John, and were baptized, through penitence were loosed from the bands of their sins by believing in Christ. John then in this way loosed the shoe-latchet of all the others, that is, the bands of sin. But Christ's shoe-latchet he was not able to unloose, because he found no sin in Him.

(non occ.) He said this to do away with the opinion of the crowd, who thought that he was the Christ; but he announces that Christ is mightier than he, who was to remit sins, which he himself could not do.