From a commentary on the gospel of John by Saint Cyril of Alexandria, bishop
If I do not go away, the Comforter will not come to you

After Christ had completed his mission on earth, it still remained necessary for us to become sharers in the divine nature of the Word. We had to give up our own life and be so transformed that we would begin to live an entirely new kind of life that would be pleasing to God. This was something we could do only by sharing in the Holy Spirit.

It was most fitting that the sending of the Spirit and his descent upon us should take place after the departure of Christ our Savior. As long as Christ was with them in the flesh, it must have seemed to believers that they possessed every blessing in him; but when the time came for him to ascend to his heavenly Father, it was necessary for him to be united through his Spirit to those who worshipped him, and to dwell in our hearts through faith. Only by his own presence within us in this way could he give us confidence to cry out, Abba, Father, make it easy for us to grow in holiness and, through our possession of the all-powerful Spirit, fortify us invincibly against the wiles of the devil and the assaults of men.

It can easily be shown from examples both in the Old Testament and the New that the Spirit changes those in whom he comes to dwell; he so transforms them that they begin to live a completely new kind of life. Saul was told by the prophet Samuel: The Spirit of the Lord will take possession of you, and you shall be changed into another man. Saint Paul writes: As we behold the glory of the Lord with unveiled faces, that glory, which comes from the Lord who is the Spirit, transforms us all into his own likeness, from one degree of glory to another.

Does this not show that the Spirit changes those in whom he comes to dwell and alters the whole pattern of their lives? With the Spirit within them it is quite natural for people who had been absorbed by the things of this world to become entirely other-worldly in outlook, and for cowards to become men of great courage. There can be no doubt that this is what happened to the disciples. The strength they received from the Spirit enabled them to hold firmly to the love of Christ, facing the violence of their persecutors unafraid. Very true, then, was our Savior’s saying that it was to their advantage for him to return to heaven: his return was the time appointed for the descent of the Holy Spirit.

From a treatise on John by Saint Augustine, bishop

Two kinds of Life

The Church recognizes two kinds of life as having been commended to her by God. One is a life of faith, the other a life of vision; one is a life passed on pilgrimage in time, the other in a dwelling place in eternity; one is a life of toil, the other of repose; one is spent on the road, the other in our homeland; one is active, involving labor, the other contemplative, the reward of labor.

The first kind of life is symbolized by the apostle Peter, the second by John. All of the first life is lived in this world, and it will come to an end with this world. The second life will be imperfect till the end of this world, but it will have no end in the next world. And so Christ says to Peter: Follow me; but of John he says: If I wish him to remain until I come, what is that to you? Your duty is to follow me.

You are to follow me by imitating my endurance of transient evils; John is to remain until my coming, when I will bring eternal blessings. A way of saying this more clearly might be: Your active life will be perfect if you follow the example of my passion, but to attain its full perfection John’s life of contemplation must wait until I come.

Perfect patience is to follow Christ faithfully, even to death, but for perfect knowledge we must await his coming. Here, in the land of the dying, the sufferings of the world must be endured; there, in the land of the living, shall be seen the good things of the Lord.

Christ’s words, I wish him to remain until I come, should not be taken to imply that John was to remain on earth until Christ’s coming, but rather that he was to wait because it is not now but only when Christ comes that the life he symbolizes will find fulfillment. On the other hand, Christ says to Peter: Your duty is to follow me, because the life Peter symbolizes can attain its goal only by action here and now.

Yet we should make no mental separation between these great apostles. Both lived the life symbolized by Peter; both were to attain the life symbolized by John. Symbolically, one followed, the other remained, but living by faith they both endured the sufferings of this present life of sorrow and they both longed for the joys of the future life of happiness.

Nor were they alone in this. They were one with the whole Church, the bride of Christ, which will in time be delivered from the trials of this life and live for ever in the joy of the next. These two kinds of life were represented respectively by Peter and John, yet both apostles lived by faith in this present, passing life and in eternal life both have the joy of vision.

And so for the sake of all the saints inseparably united to the body of Christ, to bide them through the storms of this life, Peter, the chief of the apostles, received the keys of the kingdom of heaven with the power to bind and loose sins; and for the sake of those same saints, to plumb the depths of that other, hidden life, John the evangelist reclined on the breast of Christ.

For it is not only Peter but the whole Church that binds and looses from sin; and as for the sublime teaching of John about the Word, who in the beginning was God with God, and everything else he told us about Christ’s divinity, and about the trinity and unity of the Godhead, which now, until the Lord comes, is all like a faint reflection in a mirror, but which will be seen face to face in the kingdom of heaven—it was not only John who drank in this teaching that came forth from the Lord’s breast as from a fountain. All who belong to the Lord are to drink it in, each according to his capacity, and this is why the Lord himself has spread John’s gospel throughout the world.