Prophecy of Peter's Martyrdom.
18. Thou shalt stretch forth thy hands (upon the arms of the cross), and another (i.e. the executioner) shall gird thee (viz. with the loin-cloth, the only garment allowed to criminals at their execution) and carry thee whither thou wouldest not (viz. to execution). St. John here assumes the manner of St. Peter's death to be known to his readers. According to the probably true tradition, St. Peter and St. Paul were martyred at Rome about 68 a.d., the former being crucified, the latter beheaded.
Related Commentaries of John 21:18
John 21:18-20
18 Verily, verily, I say unto thee, When thou wast young, thou girdedst thyself, and walkedst whither thou wouldest: but when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and carry thee whither thou wouldest not.
19 This spake he, signifying by what death he should glorify God. And when he had spoken this, he saith unto him,Follow me.
20 Then Peter, turning about, seeth the disciple whom Jesus loved following; which also leaned on his breast at supper, and said, Lord, which is he that betrayeth thee?