1 When the Lord Jesus had foretold His disciples the persecutions they would have to suffer after His departure, He went on to say: “And these things I said not unto you at the beginning, because I was with you; but now I go my way to Him that sent me.” And here the first thing we have to look at is, whether He had not previously foretold them of the sufferings that were to come. And the three other evangelists make it sufficiently clear that He had uttered such predictions prior to the approach of the supper: which was over, according to John, when He spoke, and added, “And these things I said not unto you at the beginning, because I was with you.”
Are we, then, to settle such a question in this way, that they, too, tell us that He was near His passion when He said these things? Then it was not when He was with them at the beginning that He so spoke, for He was on the very eve of departing, and proceeding to the Father: and so also, even according to these evangelists, it is strictly true what is here said, “And these things I said not unto you at the beginning.” But what are we to do with the credibility of the Gospel according to Matthew, who relates that such announcements were made to them by the Lord, not only when He was on the eve of sitting down with His disciples to the passover supper, but also at the beginning, when the twelve apostles are for the first time expressed by name, and sent forth on the work of God? What, then, is the meaning of what He says here, “And these things I said not unto you at the beginning, because I was with you;” but that what He says here of the Holy Spirit who was to come to them, and to bear witness, when they should have such ills to endure, this He said not unto them at the beginning, because He was with themselves?
Source: Tractates on the Gospel of John (New Advent)