3 And then, in bringing to a close this weighty and protracted discourse, He said, “These things have I spoken unto you, that in me ye might have peace. In the world you shall have tribulation; but be of good cheer, I have overcome the world.” The beginning of such tribulation was to be found in that whereof, in order to show that they were infants, to whom, as still wanting in intelligence, and mistaking one thing for another, all the great and divine things He had said were little better than proverbs, He had previously said, “Do ye now believe?
Behold, the hour comes, yea, is now come, that you shall be scattered, every man to his own.” Such, I say, was the beginning of the tribulation, but not in the same measure of their perseverance. For in adding, “and you shall leave me alone,” He did not mean that they would be of such a character in the subsequent tribulation, which they should have to endure in the world after His ascension, as thus to desert Him; but that in Him they should have peace by still abiding in Him.
But on the occasion of His apprehension, not only did they outwardly abandon His bodily presence, but they mentally abandoned their faith. And to this it is that His words have reference, “Do ye now believe? Behold, the hour comes, that you shall be scattered to your own, and shall leave me:” as if He had said, You will then be so confounded as to leave behind you even what you now believe. For they fell into such despair and such a death, so to speak, of their old faith, as was apparent in the case of Cleophas, who, after His resurrection, unaware that he was speaking with Himself, and narrating what had befallen Him, said, “We trusted that it had been He who should have redeemed Israel.” That was the way in which they then left Him, abandoning even the very faith wherewith they had formerly believed in Him.
But in that tribulation, which they encountered after His glorification and they themselves had received the Holy Spirit, they did not leave Him: and though they fled from city to city, from Himself they did not flee; but in order that, while having tribulation in the world, they might have peace in Him, instead of being fugitives from Him, it was rather Himself that they made their refuge. For in receiving the Holy Spirit, there was wrought in them the very state described to them now in the words, “Be of good cheer, I have overcome the world.”
They were of good cheer, and they conquered. But in whom, save in Him? For He had not overcome the world, were it still to overcome His members. Hence said the apostle, “Thanks be unto God, who gives us the victory;” and immediately added, “through our Lord Jesus Christ:” through Him who had said to His own, “Be of good cheer, I have overcome the world.”
Source: Tractates on the Gospel of John (New Advent)