I believe a man died and returned to life.
In more detail, there was a Jewish teacher, Jesus of Nazareth, approximately two millenia ago, who claimed to be God, upsetting the religious leaders of the day. These leaders had him executed: flogged to near the point of death, asphyxiated by hanging, and then, to be completely sure he was dead, put a spear through his heart. Now, if that was the end of the story, there would be nothing much exceptional about it – there have been other similar historical incidents. However, I believe that this man, Jesus of Nazareth, was literally returned from the dead, to physical life in a physical body. There are eyewitness testimonies to this fact that have survived to the present day, with orders of magnitude more reliability of transmission than other documents of similar age. Other historical witnesses state that these eyewitnesses held to their statement, even being executed themselves rather than renounce it. It is, however, rather audacious to posit that a man literally came back to life. Science cannot explain it, and really the only explanation that fits is that Jesus was, in fact, God, as he claimed. Once the familiarity of this claim (at least in Western culture) is put aside, the implications really are enormous.