"After that he was seen by over five hundred brethren at once, of whom the greater part remain to the present, but some have fallen asleep." (I Corinthians 15:6)
Some Christians make a big deal out of this verse, claiming that it lends strong support for the historicity of Jesus' resurrection. After all, 500 eyewitnesses should be more than enough to prove that such an event occurred beyond a shadow of a doubt, right? Here's what prominent apologist Josh McDowell says about this:
"Let's take the more than 500 witnesses who saw Jesus alive after His death and burial, and place them in a courtroom. Do you realize that if each of those 500 people were to testify for only six minutes, including cross-examination, you would have an amazing 50 hours of firsthand testimony? Add to this the testimony of many other eyewitnesses and you would well have the largest and most lopsided trial in history."
But there's an immense problem with this line of reasoning: we don't have 50 hours of firsthand testimony. In fact, we don't have the testimony of 500 people at all. We have the testimony of one person – Paul – who claims that there were 500 witnesses. He could easily be exaggerating, lying, or simply misinformed.
Could early Christians have investigated for themselves whether Paul's claim was true? Possibly, but just as most Christians today care nothing for evidence, the vast majority of Christians back then would have been perfectly happy just to take Paul's word for it. On top of that, note that Paul doesn't mention who these people are, nor where they're currently living. If Christians wanted to confirm Paul's claim, they would have had to know both their identities and locations. Then they would have had to set out on a long journey from Corinth, hundreds of miles across the ancient world, just to interview them.
Notice also that "some have fallen asleep" – in other words, died. These alleged witnesses would have been very old, so there was only a small window of opportunity for some dedicated skeptic to confirm what Paul said. And let's imagine for a moment that a skeptic found the Christians Paul was referring to, and they all said they had never seen the risen Christ – Paul was either bluffing or honestly mistaken. If that skeptic went around proclaiming that Paul was wrong, who would the other Christians believe? Most if not all of them would ignore this evidence because they likely viewed Paul as more trustworthy, and because confirmation bias and wishful thinking would cause them to cling to their existing beliefs. It would be very easy for Paul to make this claim without it coming back to haunt him.
Finally, the nail in the coffin of this supposed evidence. In verse 3, shortly before Paul mentions the witnesses, he says:
"For I delivered to you first of all that which I also received, that..."
In other words, Paul isn't even claiming that he talked to the witnesses himself; he received this information from someone else. It may even have been simply taken from the oral tradition of the time. For every link added to the chain, the chances of exaggeration or outright fabrication are compounded tremendously.
Far from being having the incontrovertible testimony of 500 witnesses, we instead have the (at least) thirdhand testimony of a single person. This evidential bottleneck comes up time and time again when critically examining the Bible. It would make little difference if Paul said there were five thousand witnesses: any claim must be considered in terms of the immediate source.