7 But wherefore these so great evils? By what desert? Hear by what desert. “For without cause have they hid for me the corruption of their trap”. For Him that is our Head, observe, the Jews did this: they hid the corruption of their trap. For whom hid they their trap? For Him who saw the hearts of those that hid. But yet was He among them like one ignorant, as though He were deceived, whereas they were in that deceived, that they thought Him to be deceived. For therefore was He as though deceived, living among them, because we among such as they were so to live, as to be without doubt deceived.
He saw His betrayer, and chose him the more to a necessary work. By his evil He wrought a great good: and yet among the twelve was he chosen, lest even the small number of twelve should be without one evil. This was an example of patience to us, because it was necessary that we should live among the evil: it was necessary that we should endure the evil, either knowing them or knowing them not: an example of patience He gave you lest you should fail, when you have begun to live among the evil. And because that School of Christ in the twelve failed not, how much more ought we to be firm, when in the great Church is fulfilled what was predicted of the mixture of the evil....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)