Peter publicly testified to his conviction that Jesus resurrected from the dead, and we have no reason to think he was lying. (It is worth noting that Peter proclaimed Jesus's resurrection with an intense boldness and confidence, and attributed this confidence to a particular life changing experience: having seen the resurrected Jesus appear to him.)1
Acts 4:13, 19 -- Now as [certain Jewish rulers, including Annas, Caiaphas, John, and Alexander] observed the confidence of Peter and John [at their hearing]… they commanded them not to speak or teach at all in the name of Jesus. But Peter and John answered… “…by the name of Jesus Christ the Nazarene, whom you crucified, whom God raised from the dead. …we cannot stop speaking about what we have seen and heard.”
Peter bet his life itself on the truth of Jesus's resurrection. This is relevant because people are disinclined to make these sorts of sacrifices for things they don't believe in.
Peter bet just about everything valuable in his life (his own comfort, time, physical well-being, etc.) on the truth of Jesus's resurrection. This is relevant because people are disinclined to make these sorts of sacrifices for things they don't believe in.