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.
3. “But now I go my way to Him that sent me; and none of you,” He says, “asks me, Where are You going?” He means that His departure would be such that none would ask Him of that which they should see taking place in broad daylight before their eyes: for previously to this they had asked Him whither He was going, and had been answered that He was going whither they themselves could not then come. Now, however, He promises that He will go away in such a manner that none of them shall ask Him whither He goes. For a cloud received Him when He ascended up from their side; and of His going into heaven they made no verbal inquiry, but had ocular evidence.
4. “But because I have said these things unto you,” He adds, “sorrow has filled your heart.” He saw, indeed, what effect these words of His were producing in their hearts; for having not yet within them the spiritual consolation, which they were afterwards to have by the Holy Spirit, what they still saw objectively in Christ they were afraid of losing; and because they could have no doubt they were about to lose Him whose announcements were always true, their human feelings were saddened, because their carnal view of Him was to be left a blank. But He knew what was most expedient for them, because that inward sight, wherewith the Holy Spirit was yet to comfort them, was undoubtedly superior; not by bringing a human body into the bodies of those who saw, but by infusing Himself into the hearts of those who believed. And then He adds, “Nevertheless I tell you the truth, it is expedient for you that I go away. For if I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you:” as if He had said, It is expedient for you that this form of a servant be taken away from you; as the Word made indeed flesh I dwell among you; but I would not that you should continue to love me carnally, and, content with such milk, desire to remain infants always. “It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you.” If I withdraw not the tender nutriment wherewith I have nourished you, you will acquire no keen relish of solid food; if you adhere in a carnal way to the flesh, you will not have room for the Spirit. For what is this, “If I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you”? Was it that He could not send Him while located here Himself? Who would venture to say so? Neither was it, that where He was, thence the Other had withdrawn, or that He had so come from the Father as that He did not still abide with the Father. And still further, how could He, even when having His own abode on earth, be unable to send Him, who we know came and remained upon Him at His baptism; yea, more, from whom we know that He was never separable? What does it mean, then, “If I go not away, the Comforter will not come unto you;” but that you cannot receive the Spirit so long as you continue to know Christ after the flesh? Hence one who had already been made a partaker of the Spirit says, “Though we have known Christ after the flesh, yet now henceforth know we [Him] no more.” For now even the very flesh of Christ he did not know in a carnal way, when brought to a spiritual knowledge of the Word that had been made flesh. And such, doubtless, did the good Master wish to intimate, when He said, “If I go not away, the Comforter will not come unto you; but if I depart, I will send Him unto you.”
Source: Tractates on the Gospel of John (New Advent)