John 21:18-19

Verily I say unto thee, When thou wast young, &c. Peter being thus restored to the apostolical office and dignity, from which he had fallen by openly denying his Master three several times, Jesus proceeded to forewarn him of the persecutions to which he in particular would be exposed in the execution of his office; intending thereby to inspire him with courage and constancy. When thou wast young thou girdedst thyself, &c. Our Lord seems to speak thus in allusion to the strength and activity which he had now showed in swimming ashore after he had girded his fisher's coat upon him. But when thou shalt be old He lived about thirty-six years after this; thou shalt stretch forth thy hands To be nailed to the cross; and another shall gird thee Such as were condemned to be crucified, were tied to the cross till the nails were driven in; and shall carry thee With the cross; whither thou wouldest not According to nature: to the place where the cross was to be set up. In other words, Instead of that liberty which in thy youth thou enjoyedst, thou shalt in thine old age be bound and carried to prison and to death. Accordingly, the evangelist adds, This spake he, signifying by what death he should glorify God Namely, that he should suffer martyrdom, and die with his hands stretched out on a cross. Observe, reader, 1st, It is not only by acting, but also and especially by suffering, that the saints glorify God. 2d, That with regard to death, which we must all suffer, it is the great concern of every good man, whatever death he dies, to glorify God in it. And when we die patiently, submitting to the will of God; die cheerfully, rejoicing in hope of the glory of God; and die usefully, witnessing to the truth and goodness of religion, and encouraging others, we glorify God in dying. 3d, That the death of the martyrs was, in a special manner, for the glorifying of God. The truths of God, which they died in defence of, were hereby confirmed; the grace of God, which carried them with so much constancy through their sufferings, was hereby magnified; and the consolations of God, which abounded toward them in their sufferings, and his promises, the springs of their consolations, have been hereby recommended to the faith and joy of all the saints. When he had spoken this, he saith, Follow me That is, as I now walk along, and show thereby that thou art willing to conform to my example, and to follow me, even to the death of the cross. Agreeably to this, the unanimous testimony of antiquity assures us that Peter was crucified.