2 The Comforter then, or Advocate (for both form the interpretation of the Greek word, paraclete), had become necessary on Christ's departure: and therefore He had not spoken of Him at the beginning, when He was with them, because His own presence was their comfort; but on the eve of His own departure it behooved Him to speak of His coming, by whom it would be brought about that with love shed abroad in their hearts they would preach the word of God with all boldness; and with Him inwardly bearing witness with them of Christ, they also should bear witness, and feel it to be no cause of stumbling when their Jewish enemies put them out of the synagogues, and slew them, with the thought that they were doing God service; because the charity bears all things, which was to be shed abroad in their hearts by the gift of the Holy Spirit. In this, therefore, is the whole meaning to be found, that He was to make them His martyrs, that is, His witnesses through the Holy Spirit; so that by His effectual working within them, they would endure the hardships of all kinds of persecution, and, set aglow at that divine fire, lose none of their warmth in the love of preaching. “These things,” therefore, He says, “have I told you, that, when their time shall come, you may remember that I told you of them”. These things, I say, I have told you, not merely because you shall have to endure such things, but because, when the Comforter has come, He shall bear witness of me, that you may not keep them back through fear, and by whom you yourselves shall also be enabled to bear witness. “And these things I said not unto you at the beginning, because I was with you,” and I myself was your comfort through my bodily presence exhibited to your human senses, and which, as infants, you were able to comprehend.
Source: Tractates on the Gospel of John (New Advent)