1 It is no light question, brethren, that meets us in the Gospel of the blessed John, when he says: “When Jesus had thus said, He was troubled in spirit, and testified, and said, Verily, verily, I say unto you, that one of you shall betray me.” Was it for this reason that Jesus was troubled, not in flesh, but in spirit, that He was now about to say, “One of you shall betray me”? Did this occur then for the first time to His mind, or was it at that moment suddenly revealed to Him for the first time, and so troubled Him by the startling novelty of so great a calamity?
Was it not a little before that He was using these words, “He that eats bread with me will lift up his heel against me”? And had He not also, previously to that, said, “And you are clean, but not all”? Where the evangelist added, “For He knew who should betray Him:” to whom also on a still earlier occasion He had pointed in the words, “Have not I chosen you twelve, and one of you is a devil?” Why is it, then, that He “was now troubled in spirit,” when “He testified, and said, Verily, verily, I say unto you, that one of you shall betray me”?
Was it because now He had so to mark him out, that he should no longer remain concealed among the rest, but be separated from the others, that therefore “He was troubled in spirit”? Or was it because now the traitor himself was on the eve of departing to bring those Jews to whom he was to betray the Lord, that He was troubled by the imminency of His passion, the closeness of the danger, and the swooping hand of the traitor, whose resolution was foreknown? For some such cause it certainly was that Jesus “was troubled in spirit,” as when He said, “Now is my soul troubled; and what shall I say?
Father, save me from this hour; but for this cause came I unto this hour.” And accordingly, just as then His soul was troubled as the hour of His passion approached; so now also, as Judas was on the point of going and coming, and the atrocious villainy of the traitor neared its accomplishment, “He was troubled in spirit.”
Source: Tractates on the Gospel of John (New Advent)