Updated 2/19/10: I have changed my assessment of the possibility that Luke recorded the appearance to the 500. Originally I had argued that it was unlikely on the grounds that Jesus’ final appearance began indoors, and only then proceeded outdoors (thus the gathering had to be small). But it occurred to me that this conclusion fails to take into account the fact that Luke is obviously telescoping his account of Jesus’ resurrection appearances into a single appearance. The appearance recorded in Luke 24:50-51 cannot be a continuation of the appearance recorded in Luke 24:36-49, and thus there is no reason to believe the latter appearance began indoors (as the former appearance obviously did).
How do I know Luke must be speaking of at least two different appearances, but telescoped them into one for his narrative? In Acts 1 Luke recounts Jesus’ final appearance and ascension from Bethany/Mount Olivet, providing much more detail than he did at the end of his Gospel. Luke declared that Jesus appeared to the apostles many times over a period of 40 days, after which He ascended into heaven (Acts 1:3-12). It is clear that the appearance in Luke 24:36-49 occurred the day of Jesus’ resurrection. If Jesus did not ascend until 40 days later, then the appearance and ascension recorded by Luke in 24:50-51 must have been separated from the appearance in 24:36-49 by 40 days. That means the appearance in 24:50-51 may have been instantiated outdoors, and thus it is possible that a group of more than 500 people could have been present. The text below has been updated to reflect my change of mind.
Matthew records a very peculiar event in connection with Jesus’ resurrection appearances. He writes, “The eleven disciples went away into Galilee, into a mountain where Jesus had appointed them. And when they saw him they worshipped him, but some doubted” (Mt 28:16-17). It is highly unlikely that Matthew would have invented a story in which individuals who see the resurrected Christ for themselves still do not believe in Jesus. This comment, then, lends credibility to the historicity of Matthew’s report.
But the question remains: Who doubted? Was it some of the 11 apostles, or members of another unidentified group? It’s unlikely that some of the apostles doubted. The “they” in verse 17 most naturally refers to all 11 disciples mentioned in verse 16, and thus it stands to reason that all 11 worshipped Jesus: when “they” (the Eleven) saw Him “they” (the Eleven) worshipped Him. The real clincher, however, is that we know this was not Jesus’ first appearance to the Eleven. His first appearance to them was in Jerusalem on the eve of His resurrection (Mk 16:14; Lk 24:36-42; Jn 20:19-23). Since that appearance convinced the Eleven that Jesus had risen, they cannot be numbered with those who doubted at Jesus’ Galilean appearance.
Who were those that doubted, then, if it was not some of the Eleven? Most likely, they were members of a larger, unidentified group of witnesses that accompanied the Eleven to the mountain, all of whom were disciples of Jesus during His ministry. In fact, there is good reason to speculate that this was the epiphany Paul speaks of in 1 Corinthians 15:6 wherein Jesus appeared to more than 500 people at once.