1 In the words preceding this chapter of the Gospel, the Lord strengthened His disciples to endure the hatred of their enemies, and prepared them also by His own example to become the more courageous in imitating Him: adding the promise, that the Holy Spirit should come to bear witness of Him, and also that they themselves could become His witnesses, through the effectual working of His Spirit in their hearts. For such is His meaning when He says, “He shall bear witness of me, and you also shall bear witness.” That is to say, because He shall bear witness, you also shall bear witness: He in your hearts, you in your voices; He by inspiration, you by utterance: that the words might be fulfilled, “Their sound has gone forth into all the earth.” For it would have been to little purpose to have exhorted them by His example, had He not also filled them with His Spirit. Just as we see that the Apostle Peter, after having heard His words, when He said, “The servant is not greater than his lord: if they have persecuted me, they will also persecute you;” and seen that already fulfilled in Him, wherein, had example been sufficient, he ought to have imitated the patient endurance of his Lord, yet succumbed and fell into denial, as utterly unable to bear what He saw his Master enduring. But when he really received the gift of the Holy Spirit, he preached Him whom he had denied; and whom he had been afraid to confess, he had no fear now in openly proclaiming. Already, indeed, had he been sufficiently taught by example to know what was proper to be done; but not yet was he inspired with the power to do what he knew: he had got instruction to stand, but not the strength to keep him from falling. But after this was supplied by the Holy Spirit, he preached Christ even to the death, whom, in his fear of death, he had previously denied. And so the Lord in this succeeding chapter, on which we have now to address you, says, “These things have I spoken unto you, that you should not be offended.” As it is sung in the psalm, “Great peace have they who love Your law, and nothing shall offend them.” Properly enough, therefore, with the promise of the Holy Spirit, by whose operation in their hearts they should be made His witnesses, He added, “These things have I spoken unto you, that you should not be offended.” For when the love of God is shed abroad in our hearts by the Holy Spirit given unto us, they have great peace who love God's law, so that nothing may offend them.
2. And then He expressly declares what they were to suffer: “They shall put you out of the synagogues.” But what harm was it for the apostles to be expelled from the Jewish synagogues, as if they were not to separate themselves therefrom, although no one expelled them? Doubtless He meant to announce with reprobation, that the Jews would refuse to receive Christ, from whom they as certainly would refuse to withdraw; and so it would come to pass that the latter, who could not exist without Him, would also be cast out along with Him by those who would not have Him as their place of abode. For certainly, as there was no other people of God than that seed of Abraham, they would, had they only acknowledged and received Christ, have remained as the natural branches in the olive tree; nor would the churches of Christ have been different from the synagogues of the Jews, for they would have been one and the same, had they also desired to abide in Him. But having refused, what remained but that, continuing themselves out of Christ, they put out of the synagogues those who would not abandon Christ? For having received the Holy Spirit, and so become His witnesses, they would certainly not belong to the class of whom it is said: “Many of the chief rulers of the Jews believed on Him; but for fear of the Jews they dared not confess Him, lest they should be put out of the synagogue: for they loved the praise of men more than the praise of God.” And so they believed on Him, but not in the way He wished them to believe when He said: “How can you believe, who expect honor one of another, and seek not the honor that comes from God only?” It is, therefore, with those disciples who so believe in Him, that, filled with the Holy Spirit, or, in other words, with the gift of divine grace, they no longer belong to those who, “ignorant of the righteousness of God, and going about to establish their own, have not submitted themselves to the righteousness of God;” nor to those of whom it is said, “They loved the praise of men more than the praise of God:” that the prophecy harmonizes, which finds its fulfillment in their own case: “They shall walk, O Lord, in the light of Your countenance: and in Your name shall they rejoice all the day; and in Your righteousness shall they be exalted: for You are the glory of their strength.” Rightly enough is it said to such, “They shall cast you out of the synagogues;” that is, they who “have a zeal for God, but not according to knowledge;” because, “ignorant of God's righteousness, and going about to establish their own,” they expel those who are exalted, not in their own righteousness, but in God's, and have no cause to be ashamed at being expelled by men, since He is the glory of their strength.
Source: Tractates on the Gospel of John (New Advent)