John Calvin 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)

Signifying by what death he should glorify God. This circumlocution is highly emphatic. For though the goal presented to all believers should be to glorify God both by their life and by their death, yet John intended to offer a remarkable commendation to honor the death of those who, by their blood, seal the Gospel of Christ and glorify his name, as Paul teaches us (Philippians 1:20). It is now our duty to reap the fruit which the death of Peter has yielded; for it should be attributed to our indolence if our faith is not confirmed by it, and if we do not keep the same goal in view, so that the glory of God may be displayed by us. If the Papists had considered this purpose in the death of the martyrs, that sacrilegious and detestable invention would never have entered their minds: that their death contributes to appease the wrath of God and to pay the ransom for our sins.

And when he had said this. Christ here explains what was the purpose of that prediction of a violent death. This was so that Peter might be prepared to endure it, as if Christ had said, “Since you must endure death by my example, follow your leader.” Again, so that Peter might more willingly obey God who calls him to the cross, Christ offers himself as a leader; for this is not a general exhortation in which Christ invites Peter to imitate him (Christ), but Christ speaks only of the kind of death. Now, this single consideration greatly soothes all the bitterness of death, when the Son of God presents himself before our eyes with his blessed resurrection, which is our triumph over death.