5 Accordingly it remains that if we are to believe that the Lord Jesus, in this prayer, prayed for all of His own who either then were or should thereafter be in this life, which is a state of trial upon earth, we must so understand the expression, “through their word,” as to believe that it here signified the word of faith itself which they preached in the world, and that it was called their word because it was primarily and principally preached by them. For it was already in the course of being preached by them in the earth when Paul received that same word of theirs by the revelation of Jesus Christ.
Whence also it came about that he compared the Gospel with them, lest by any means he had run, or should run, in vain; and they gave him their right hand because in him also they found, although not given him by them, their own word which they were already preaching, and in which they were now established. And in regard to this word of the resurrection of Christ, it is said by the same apostle, “Whether it were I, or they, so we preach, and so ye believed;” and again, “This is the word of faith,” he says, “which we preach, that if you shall confess with your mouth that Jesus is the Lord, and shall believe in your heart that God has raised Him from the dead, you shall be saved.” And in the Acts of the Apostles we read that in Christ, God has marked out [the ground of] faith unto all men, in that He has raised Him from the dead. Accordingly, this word of faith, because principally and primarily preached by the apostles who adhered to Him, was called their word.
Not, however, on that account does it cease to be the word of God because it is called their word; for the same apostle says that the Thessalonians received it from him “not as the word of men, but, as it is in truth, the word of God.” “Of God,” for the very reason that it was freely given by God; but called “their word,” because primarily and principally committed to them by God to be preached. In the same way also the thief mentioned above had in the matter of his own faith their word, which was called theirs precisely because the preaching of it primarily and principally pertained to the office they filled.
And once more, when murmuring arose among the Grecian widows in reference to the serving of the tables, previous to the time when Paul was brought to the faith of Christ, the reply given by the apostles, who before then had adhered to the Lord, was: “It is not good that we should leave the word of God, and serve tables.” Then it was that they provided for the ordination of deacons, that they themselves might not be drawn aside from the duty of preaching the word. Hence that was properly enough called their word which is the word of faith, whereby all, from whatever quarter they had heard it, believed on Christ, or, as yet to hear it, should thereafter believe.
In this prayer, therefore, all whom He redeemed, whether then alive or thereafter to live in the flesh, were prayed for by our Redeemer when, praying for the apostles who were then with Him, He also conjoined those who were yet to believe in Him through their word. But what, after such conjunction, He then proceeds to say, must be reserved for discussion in another discourse.
Source: Tractates on the Gospel of John (New Advent)