42 Again the Gospels fill up what is lacking in one another: we learn some things from one, some from another, and so on, because all are the proclamation of the same spirit. Thus John, who especially brings out the working of spiritual causes in the Gospel, preserves this prayer of the Lord for the Apostles, which all the others passed over: how He prayed, namely, Holy Father, keep them in Your Name....while I was them I kept them in Your Name: those whom You gave Me I have kept. That prayer was not for Himself but for His Apostles; nor was He sorrowful for Himself, since He bids them pray that they be not tempted; nor is the angel sent to Him, for He could summon down from Heaven, if He would, twelve thousand angels; nor did He fear because of death when He was troubled unto death.
Again, He does not pray that the cup may pass over Himself, but that it may pass away from Himself, though before it could pass away He must have drunk it. But, further, 'to pass away' does not mean merely 'to leave the place,' but 'not to exist any more at all:' which is shown in the language of the Gospels and Epistles: for example, Heaven and earth shall pass away, but My word shall not perish: also the Apostle says, Behold the old things are passed away; they have become new. And again, The fashion of this world shall pass away. The cup, therefore, of which He prays to the Father, cannot pass away unless it be drunk; and when He prays, He prays for those whom He preserved, so long as He was with them, whom He now hands over to the Father to preserve.
Now that He is about to accomplish the mystery of death He begs the Father to guard them. The presence of the angel who was sent to Him (if this explanation be true) is not of doubtful significance. Jesus showed His certainty that the prayer was answered when, at its close, He bade the disciples sleep on. The effect of this prayer and the security which prompted the command, 'sleep on,' is noticed by the Evangelist in the course of the Passion, when he says of the Apostles just before they escaped from the hands of the pursuers, That the word might be fulfilled which He had spoken, Of those whom You have given Me I lost not one of them. He fulfils Himself the petition of His prayer, and they are all safe; but He asks that those whom He has preserved the Father will now preserve in His own Name.
And they are preserved: the faith of Peter does not fail: it cowered, but repentance followed immediately.
Source: On the Trinity (New Advent)