What reverence is due to Him he then teaches us by his own humility; going on to say: the latchet of whose shoe I am not worthy to loose…!
[...] Who does not know that sandals are made from the skins of dead animals? The Lord, in becoming Incarnate, appears among men, as though shod; because over His Divinity, he has put on as it were the mortal covering of our corruptibility.
Hence also the prophet says: Into Edom will I stretch out my shoe (Ps. 15:10). The Gentiles are signified by Edom; His assumed mortality by the shoe.
The Lord therefore declares that He extends His shoe into Edom, because through the flesh He became known to the Gentiles; as if the Divinity had come to us with feet shod.
But the human eye does not suffice to penetrate the mystery of this incarnation. For in no way may we search out how the Word became embodied; how the Supreme Life-Giving Spirit, was quickened within the womb of a mother; how That Which has no beginning was both conceived and came into existence.
The latchets of His shoe are therefore the seals of a mystery. John was not worthy to loose His shoe, because he was unable to search into the mystery of His Incarnation.
What then does he mean when he says, the latchet of whose shoe I am not worthy to loose, except openly and humbly to confess his ignorance?
It is as though he were to say: what wonder that He is preferred before me, Whom I know to be born after me, but the Mystery of Whose Birth I am unable to comprehend.
Behold John, filled with the Spirit of prophecy, shining with knowledge, yet he plainly declares that as to this mystery he knows nothing.
In this connection, Dearest Brethren, we should note and ponder with careful thought, how holy men of God, in order to safeguard themselves in humility, when they know many things well, endeavour to keep before their minds that which they do not know.
Thus on the one hand, they remind themselves of their own limitations, and on the other, they are not raised above themselves because of those things in which their mind is accomplished.
Knowledge indeed is virtue, but humility is the guardian of virtue. For the future then, let you be humble in your minds with regard to whatever you may know, lest what the virtue of knowledge has stored, the wind of vanity may carry off.
Gregory the Great (c.540-604): Homilies on the Gospels, @ Lectionary Central.