10 Then said the Lord Jesus: “Have not I chosen you twelve, and one of you is a devil?” Therefore, should He have said, “I have chosen eleven:” or is a devil also chosen, and among the elect? Persons are wont to be called “elect” by way of praise: or was man elected because some great good was done by him, without his will and knowledge? This belongs peculiarly to God; the contrary is characteristic of the wicked. For as wicked men make a bad use of the good works of God; so, on the contrary, God makes a good use of the evil works of wicked men.
How good it is that the members of the body are, as they can be disposed only by God, their author and framer! Nevertheless what evil use does wantonness make of the eyes? What ill use does falsehood make of the tongue? Does not the false witness first both slay his own soul with his tongue, and then, after he has destroyed himself, endeavor to injure another? He makes an ill use of the tongue, but the tongue is not therefore an evil thing; the tongue is God's work, but iniquity makes an ill use of that good work of God.
How do they use their feet who run into crimes? How do murderers employ their hands? And what ill use do wicked men make of those good creatures of God that lie outside of them? With gold they corrupt judgment and oppress the innocent. Bad men make a bad use of the very light; for by evil living they employ even the very light with which they see into the service of their villanies. A bad man, when going to do a bad deed, wishes the light to shine for him, lest he stumble; he who has already stumbled and fallen within; that which he is afraid of in his body has already befallen him in his heart.
Hence, to avoid the tediousness of running through them separately, a bad man makes a bad use of all the good creatures of God: a good man, on the contrary, makes a good use of the evil deeds of wicked men. And what is so good as the one God? Since, indeed, the Lord Himself said, “There is none good, but the one God.” By how much He is better, then, by so much the better use He makes of our evil deeds. What worse than Judas? Among all that adhered to the Master, among the twelve, to him was committed the common purse; to him was allotted the dispensing for the poor.
Unthankful for so great a favor, so great an honor, he took the money, and lost righteousness: being dead, he betrayed life: Him whom he followed as a disciple, he persecuted as an enemy. All this evil was Judas's; but the Lord employed his evil for good. He endured to be betrayed, to redeem us. Behold, Judas's evil was turned to good. How many martyrs has Satan persecuted! If Satan left off persecuting, we should not today be celebrating the very glorious crown of Saint Laurence.
If then God employs the evil works of the devil himself for good, what the bad man effects, by making a bad use, is to hurt himself, not to contradict the goodness of God. The Master makes use of that man. And if He knew not how to make use of him, the Master contriver would not have permitted him to be. Therefore, He says, “One of you is a devil,” while I have chosen you twelve. This saying, “I have chosen you twelve,” may be understood in this way, that twelve is a sacred number.
For the honor of that number was not taken away because one was lost, for another was chosen into the place of the one that perished. The number remained a sacred number, a number containing twelve: because they were to make known the Trinity throughout the whole world, that is, throughout the four quarters of the world. That is the reason of the three times four. Judas, then only cut himself off, not profaned the number twelve: he abandoned his Teacher, for God appointed a successor to take his place.
Source: Tractates on the Gospel of John (New Advent)