John 21:18,19

Ver 18. Verily, verily, I say to you, When you were young, you girded yourself, and walked where you would: but when you shall be old, you shall stretch forth your hands, and another shall gird you, and carry you whither you would not. 19a. This spoke he, signifying by what death he should glorify God.

CHRYS. Our Lord having made Peter declare his love, informs him of his future martyrdom; an intimation to us how we should love: Verily, verily, I say to you, When you were young, you girded yourself, and walked where you would. He reminds him of his former life, because, whereas in worldly matters a young man has powers, an old man none; in spiritual things, on the contrary, virtue is brighter, manliness stronger, in old age; age is no hindrance to grace. Peter had all along desired to share Christ's dangers; so Christ tells him, Be of good cheer; I will fulfill your desire in such a way, that what you has not suffered when young, you shall suffer when old: But when you are old. Whence it appears, that he was then neither a young nor an old man, but in the prime of life.

ORIGEN. It is not easy to find any ready to pass at once from this life; and so he says to Peter, When you are old, you shall stretch forth your hand.

AUG. That is, shall be crucified. And to come to this end, Another shall gird you, and carry you where you would not. First He said what would come to pass, secondly, how it would come to pass. For it was not when crucified, but when about to be crucified, that he was led where he would not. He wished to be released from the body, and be with Christ; but, if it were possible, he wished to attain to eternal life without the pains of death; to which he went against his will, but conquered by the force of his will, and triumphing over the human feeling, so natural a one, that even old age could not deprive Peter of it. But whatever be the pain of death, it ought to be conquered by the strength of love for Him, Who being our life, voluntarily also underwent death for us. For if there is no pain in death, or very little, the glory of martyrdom would not be great.

CHRYS. He says, Where you would not, with reference to the natural reluctance of the soul to be separated from the body; an instinct implanted by God to prevent men putting an end to themselves.

Then raising the subject, the Evangelist says, This spoke He, signifying by what death he should glorify God: not, should die: he expresses himself so, to intimate that to suffer for Christ was the glory of the sufferer. But unless the mind is persuaded that He is very God, the sight of Him can in no way enable us to endure death. Wherefore the death of the saints is certainty of divine glory.

AUG. He who denied and loved, died in perfect love for Him, for Whom he had promised to die with wrong haste. It was necessary that Christ should first die for Peter's salvation, and then Peter die for Christ's Gospel.