1 This short section of the Gospel, brethren, we have in this lesson brought forward for exposition, as thinking that we ought also to say something of the Lord's betrayer, as now plainly enough disclosed by the dipping and holding out to him of the piece of bread. Of that indeed which precedes, (namely), that Jesus, when about to point him out, was troubled in spirit, we have treated in our last discourse; but what I perhaps omitted to mention there, the Lord, by His own perturbation of spirit, thought proper to indicate this also, that it is necessary to bear with false brethren, and those tares that are among the wheat in the Lord's field until harvest-time, because that when we are compelled by urgent reasons to separate some of them even before the harvest, it cannot be done without disturbance to the Church.
Such disturbance to His saints in the future, through schismatics and heretics, the Lord in a way foretold and prefigured in Himself, when, at the moment of that wicked man Judas' departure, and of his thereby bringing to an end, in a very open and decided way, his past intermingling with the wheat, in which he had long been tolerated, He was troubled, not in body, but in spirit. For it is not spitefulness, but charity, that troubles His spiritual members in scandals of this kind; lest perchance, in separating some of the tares, any of the wheat should also be uprooted therewith.
Source: Tractates on the Gospel of John (New Advent)