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 Neither was it, that where He was, thence the Other had withdrawn, or that He had so come from the Father as that He did not still abide with the Father.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)