Albert Barnes Commentary John 21:19

"Now this he spake, signifying by what manner of death he should glorify God. And when he had spoken this, he saith unto him, Follow me." — John 21:19 (ASV)

By what death. In these words two things are implied:

1. That Peter would die a violent death; and
2. That his death would be such as to honour God.

The ancients say that Peter was crucified at Rome, about thirty-four years after this, with his head downward. Clemens says that he was led to the crucifixion with his wife, and sustained her in her sufferings by exhorting her to remember the example of her Lord. He also adds that he died, not as the philosophers did, but with a firm hope of heaven, and patiently endured the pangs of the cross (Stromata, Book VII).

This declaration of the Saviour was doubtless continually before the mind of Peter, and to the hour of his death he maintained the utmost constancy and fidelity in his cause, thus justifying the appellation which the Lord Jesus gave him—a rock.