The resurrection of Jesus, that is. The resurrection is an amazing event that changed history forever. It was an event that caused people to worship a nobody from a dusty Roman outpost called Nazareth. It was an event that caused people to be willing to die rather than deny what they claimed to have seen.
As today with atheists, there were those back then who were deeply offended that human beings could be so gullible as to believe that someone had come back to life. A man known now as the apostle Paul, especially took exception to the claim of several dozen people, that Jesus of Nazareth, who had been crucified and buried, had then risen from the dead. So offended was Paul that he tortured and killed followers of the risen Christ in order to make them stop talking foolishness.
And then it happened.
Jesus, the risen Jesus, stopped Paul in his tracks. Paul encountered the risen Jesus, and Paul was forever changed. A man who already had a world-view and a powerful religion changed them both, completely, because he met and was changed by Jesus the Christ who was raised from the dead.