“But I tell you the truth: it is expedient to you that I go. For if I go not, the Paraclete will not come to you: but if I go, I will send him to you.” (Jn 16:7)
This verse, from the Gospel reading of the 4th Sunday after Easter, has an air of mystery to it.
Why does Christ say that unless He GOES the Paraclete will not come?
Why does Christ NEED to Ascend to the Father? Why couldn't he just stay here, in human form, with us?
St. Augustine gives us an answer about John 16:7 in his Letter on the Trinity
"But this is said, not on account of any inequality of the Word of God and of the Holy Spirit, but as though the presence of the Son of man with them would be a hindrance to the coming of Him, who was not less, because He did not “empty Himself, taking upon Him the form of a servant,” as the Son did. It was necessary, then, that the form of a servant should be taken away from their eyes, because, through gazing upon it, they thought that alone which they saw to be Christ. Hence also is that which is said, “If ye loved me, ye would rejoice because I said, “I go unto the Father; for my Father is greater than I:” that is, on that account it is necessary for me to go to the Father, because, whilst you see me thus, you hold me to be less than the Father through that which you see; and so, being taken up with the creature and the “fashion” which I have taken upon me, you do not perceive the equality which I have with the Father." (De Trin. 1.9)
Augustine clarifies that certainly the verse is not intended to intimate some sort of denial of the nature of the Trinity but rather that Christ needed to Ascend so that we would not know Christ ONLY as a man but as truly equal and consubstantial with the Father.
The Catechism of the Council of Trent has much of the same Augustinian reasoning when addressing the question of why Christ needed to ascend. It also beautifully relates that until Christ, in fleshly form, left our presence our love for Him would be imperfect. Once he DID leave He would be able to send the Holy Ghost, by Divine love, to help PERFECT our love for Him. In other words, His Ascension and sending of the Spirit would allow us to love Him as Fully God and not just as the humanly present Jesus.
"And, indeed, if Christ the Lord were dwelling on earth, our whole strength would be fixed upon the very sight of his human person, and the enjoyment of his presence, and we should regard only that man, who was to bestow on us such blessings, and would cherish towards him a sort of earthly affection. But, by his ascension into heaven, he has rendered our affection for him spiritual, and has made us venerate and love as God him who, though now absent, is the object of our thoughts. This we learn partly from the example of the Apostles, who, whilst our Lord was personally present with them, seemed to judge of him in some measure humanly; and in part, from these words of our Lord himself: ʻit is expedient for you that I goʻ, for that imperfect love, with which they had cherished Jesus Christ when present, was to be perfected by divine love, and that by the coming of the Holy Ghost; and, therefore, he immediately subjoins: ʻIf I go not, the Comforter will not come to you." (Roman Catechism of Trent 1.7.8)
image from the 'Public domain' via wikimedia commons. The Ascension by Benjamin West, 1801