Gregory the Great
Catena Aurea · Johannesevangelium 1
And they which were sent were of the Pharisees. And they asked him, and said unto him, Why baptizest thou then, if thou he not that Christ, nor Elias, neither that prophet? John answered them, saying, I baptize with water: but there standeth one among you, whom ye know not; He it is, who coming after me is preferred before me, whose shoe’s latchet I am not worthy to unloose. These things were done in Bethabara beyond Jordan, where John was baptizing.
(Hom. vii. in Ev. c. 3) Or thus: It was a law of the old dispensation, that, if a man refused to take the woman, who of right came to him, to wife, he who by right of relationship came next to be the husband, should unloose his shoe. Now in what character did Christ appear in the world, but as Spouse of the Holy Church? (John 3:29.) John then very properly pronounced himself unworthy to unloose this shoe’s latchet: as if he said, I cannot uncover the feet of the Redeemer, for I claim not the title of spouse, which I have no right to. Or the passage may be explained in another way. We know that shoes are made out of dead animals. Our Lord then, when He came in the flesh, put on, as it were, shoes; because in His Divinity He took the flesh of our corruption, wherein we had of ourselves perished. And the latchet of the shoe, is the seal upon the mystery. John is not able to unloose the shoe’s latchet; i. e. even he cannot penetrate into the mystery of the Incarnation. So he seems to say: What wonder that He is preferred before me, Whom, being born after me, I contemplate, yet the mystery of Whose birth I comprehend not.