1 I Know, dearly beloved, that some may be moved, as the godly to inquire into the meaning of, and the ungodly to find fault with, the statement, that it was after the Lord had given the bread, that had been dipped, to His betrayer that Satan entered into him. For so it is written: “And when He had dipped the bread, He gave it to Judas Iscariot, the Son of Simon. And after the bread, then entered Satan into him.” For they say, Was this the worth of Christ's bread, given from Christ's own table, that after it Satan should enter into His disciple?
And the answer we give them is, that thereby we are taught rather how much we need to beware of receiving what is good in a sinful spirit. For the point of special importance is, not the thing that is received, but the person that receives it; and not the character of the thing that is given, but of him to whom it is given. For even good things are hurtful, and evil things are beneficial, according to the character of the recipients. “Sin,” says the apostle, “that it might appear sin, wrought death to me by that which is good.” Thus, you see, evil is brought about by the good, so long as that which is good is wrongly received.
It is he also that says: “Lest I should be exalted unduly through the greatness of my revelations, there was given to me a thorn in my flesh, the messenger of Satan to buffet me. For which thing I besought the Lord thrice, that He would take it away from me; and He said unto me, My grace is sufficient for you: for strength is made perfect in weakness.” And here, you see, good was brought about by that which was evil, when the evil was received in a good spirit. Why, then, do we wonder if Christ's bread was given to Judas, that thereby he should be made over to the devil; when we see, on the other hand, that Paul was visited by a messenger of the devil, that by such an instrumentality he might be perfected in Christ?
In this way, both the good was injurious to the evil man, and the evil was beneficial to the good. Bear in mind the meaning of the Scripture, “Whosoever shall eat the bread or drink the cup of the Lord unworthily, shall be guilty of the body and blood of the Lord.” And when the apostle said this, he was dealing with those who were taking the body of the Lord, like any other food, in an undiscerning and careless spirit. If, then, he is thus taken to task who does not discern, that is, does not distinguish from the other kinds of food, the body of the Lord, what condemnation must be his, who in the guise of a friend comes as an enemy to His table!
If negligence in the guest is thus visited with blame, what must be the punishment that will fall on the man that sells the very person who has invited him to his table! And why was the bread given to the traitor, but as an evidence of the grace he had treated with ingratitude?
Source: Tractates on the Gospel of John (New Advent)