What changed in Peter after he witnessed the resurrection?
After witnessing the resurrection and its power, Peter became a different person. He was no longer afraid of anybody. He stood up to the Jewish religious leaders and to the Romans, and no doubt took a martyr's death because of it. Earlier, before the resurrection, he had denied Jesus three times out of fear.
Les explains that before the crucifixion, when Jesus was arrested and a young maid thought she recognized him, Peter denied being one of them three times; he even swore, because he was afraid of the powers that were. But after Peter witnessed the resurrection and the power of it, he was a different person—no longer afraid of anybody. He stood up to the Jewish religious leaders and the Romans, and no doubt took a martyr's death because of it. Later, however, as an ordinary practicing Jew in a moment of weakness among Gentile believers, he blew his testimony again, and Paul had to call him on it. Other Jews dissembled or withdrew likewise with him, and even Barnabas was carried away with their dissimulation or hypocrisy, so they could not eat with the Gentiles.