Pentakosioi (πεντακόσιοι) means five hundred and appears in its theologically decisive NT use in 1 Corinthians 15:6, where Paul declares that the risen Christ appeared to more than five hundred brothers at once, most of whom were still alive at the time of writing. This is the largest single gathering of resurrection witnesses cited in the NT.
1 Corinthians 15:3-8 is the earliest creedal statement of the resurrection, preceding the writing of any Gospel. Paul's inclusion of the pentakosioi is a deliberate apologetic move: he is essentially inviting his readers to interview the witnesses — "most of them are still alive." This is not mythology or legend; it is recent, verifiable history.
The appearance to five hundred simultaneously rules out hallucination (a psychological phenomenon that does not affect hundreds simultaneously) and legend formation (the witnesses are still alive to correct false accounts). The pentakosioi are Paul's evidence that the resurrection is a public, historical event with a crowd of eyewitnesses large enough to constitute legal proof many times over. The resurrection is the best-attested event of the ancient world.