4 All these things did that fall effect, and whereas before that he attributes all to himself, saying, “Though all men shall be offended, yet will I not be offended;” and, “If I should die, I will not deny You” (when he should have said, If I receive the assistance from You)—yet after these things altogether the contrary, “Why do ye give heed to us, as though by our own power or holiness we had made him to walk?”
Hence we learn a great doctrine, that a man's willingness is not sufficient, unless any one receive the succor from above; and that again we shall gain nothing by the succor from above, if there be not a willingness. And both these things do Judas and Peter show; for the one, though he had received much help, was profited nothing, because he was not willing, neither contributed his part; but this one, though he was ready in mind, because he received no assistance, fell. For indeed of these two things is virtue's web woven.
Wherefore I entreat you neither (when you have cast all upon God) to sleep yourselves, nor, when laboring earnestly, to think to accomplish all by your own toils. For neither is it God's will that we should be supine ourselves, therefore He works it not all Himself; nor yet boasters, therefore He did not give all to us; but having removed what was hurtful in either way, left that which is useful for us. Therefore He suffered even the chief apostle to fall, both rendering him more humbled in mind, and training him thenceforth to greater love. “For to whom more is forgiven,” it is said, “he loves more.”
Let us then in everything believe God, and gainsay Him in nothing, though what is said seem to be contrary to our thoughts and senses, but let His word be of higher authority than both reasonings and sight. Thus let us do in the mysteries also, not looking at the things set before us, but keeping in mind His sayings.
For His word cannot deceive, but our senses are easily beguiled. That has never failed, but this in most things goes wrong. Since then the word says, “This is my body,” let us both be persuaded and believe, and look at it with the eyes of the mind.
For Christ has given nothing sensible, but though in things sensible yet all to be perceived by the mind. So also in baptism, the gift is bestowed by a sensible thing, that is, by water; but that which is done is perceived by the mind, the birth, I mean, and the renewal. For if you had been incorporeal, He would have delivered you the incorporeal gifts bare; but because the soul has been locked up in a body, He delivers you the things that the mind perceives, in things sensible.
How many now say, I would wish to see His form, the mark, His clothes, His shoes. Lo! You see Him, Thou touchest Him, you eat Him. And thou indeed desirest to see His clothes, but He gives Himself to you not to see only, but also to touch and eat and receive within you.
Let then no one approach it with indifference, no one faint-hearted, but all with burning hearts, all fervent, all aroused. For if Jews standing, and having on their shoes and their staves in their hands, ate with haste, much more ought thou to be watchful. For they indeed were to go forth to Palestine, wherefore also they had the garb of pilgrims, but you are about to remove unto Heaven.
Source: Homilies on the Gospel of St. Matthew (New Advent)