Jesus said this to indicate the kind of death by which Peter would glorify God. Then he said to him, "Follow me!" (John 21:19 NIV)

Sometimes, I am of the opinion that being fabulously successful under the world's definition is how God wants me to best glorify Him. But here, John states that it is Peter's death that would glorify God (this is not to say that nothing else that Peter would do would bring glory to God). Perhaps we should devote more thought to how our death can be an opportunity for God to be glorified. And of course, the days and months and years preceding that time, all the way to the present as well.