Acts 12:17 -- But motioning to them with his hand to be silent, he described to them how the Lord had led him out of the prison. And he said, “Report these things to James and the brethren.” Then he left and went to another place.
In A.D. 97, Clement of Rome testified to Peter's sacrifices in hopes of receiving things he could only recieve if Christianity were true. This is relevant because Peter would be disinclined to make these sorts of sacrifices if he did not actually believe Christianity were true.
Clement of Rome (writing no later than c. AD 96): “…the greatest and most righteous pillars [of the Church] have been persecuted and put to death. …the illustrious apostles. Peter… endured not one or two, but numerous labours and when he had at length suffered martyrdom, departed to the place of glory due to him.” [1 Clement 5:2-7 (1st Letter to the Corinthians)]
Peter died in an unpleasant manner, most plausibly as a martyr.
John 21:18-18 -- Truly, truly, I say to you [Peter], when you were younger, you used to gird yourself and walk wherever you wished; but when you grow old, you will stretch out your hands and someone else will gird you, and bring you where you do not wish to go.” Now this He said, signifying by what kind of death he would glorify God.
All the apostles suffered and labored as if they believed.