1 The Lord Jesus, in the now close proximity of His passion, after praying for His disciples, whom He also named apostles, with whom He had partaken of that last supper from which His betrayer had taken his departure on being revealed by the sop of bread, and with whom, after the latter's departure, and before beginning His prayer in their behalf, He had already spoken at length, conjoined all others also who were yet to believe in Him, and said to the Father, “Neither pray I for these alone,” that is, for the disciples who were with Him at the time, “but for them also,” He adds, “who shall believe in me through their word.”
Whereby He wished all His own to be understood: not only such as were then in the flesh, but those also who were yet to come. For all that have since believed on Him have doubtless believed, and shall yet believe till He come, through the word of the apostles; for to themselves He had said, “And ye also shall bear witness, because you have been with me from the beginning;” and by them was the gospel ministered even before it was written, and every one assuredly who believes in Christ believes the gospel.
Accordingly, those who He says should believe in Him through their word, are not to be understood as referring only to such as heard the apostles themselves while they lived in the flesh; but others also after their decease, and we, too, born long afterwards, have believed on Christ through their word. For they that were then with Him preached to the others what they had heard from Him; and so their word, that we too might believe, has found its way to us, and wherever His Church exists, and shall yet reach down to posterity, whoever and wherever they be who shall hereafter believe in Him.
Source: Tractates on the Gospel of John (New Advent)