10 Look now, and learn that this Judas did not become perverted only at the time when he yielded to the bribery of the Jews and betrayed his Lord. For not a few, inattentive to the Gospel, suppose that Judas only perished when he accepted money from the Jews to betray the Lord. It was not then that he perished, but he was already a thief, and a reprobate, when following the Lord; for it was with his body and not with his heart that he followed. He made up the apostolic number of twelve, but had no part in the apostolic blessedness: he had been made the twelfth in semblance, and on his departure, and the succession of another, the apostolic reality was completed, and the entireness of the number conserved. What lesson then, my brethren, did our Lord Jesus Christ wish to impress on His Church, when it pleased Him to have one castaway among the twelve, but this, that we should bear with the wicked, and refrain from dividing the body of Christ?
Here you have Judas among the saints—that Judas, mark you! Who was a thief, yea— do not overlook it— not a thief of any ordinary type, but a thief and a sacrilegist: a robber of money bags, but of such as were the Lord's; of money bags, but of such as were sacred. If there is a distinction made in the public courts between such crimes as ordinary theft and peculation—for by peculation we mean the theft of public property; and private theft is not visited with the same sentence as public—how much more severe ought to be the sentence on the sacrilegious thief, who has dared to steal, not from places of any ordinary kind, but to steal from the Church?
He who thieves from the Church, stands side by side with the castaway Judas. Such was this man Judas, and yet he went in and out with the eleven holy disciples. With them he came even to the table of the Lord: he was permitted to have intercourse with them, but he could not contaminate them. Of one bread did both Peter and Judas partake, and yet what communion had the believer with the infidel? Peter's partaking was unto life, but that of Judas unto death. For that good bread was just like the sweet savor.
For as the sweet savor, so also does the good bread give life to the good, and bring death to the wicked. “For he that eats unworthily, eats and drinks judgment to himself:” “judgment to himself,” not to you. If, then, it is judgment to himself, not to you, bear as one that is good with him that is evil, that you may attain unto the rewards of the good, and be not hurled into the punishment of the wicked.
Source: Tractates on the Gospel of John (New Advent)