It would not matter if Paul had named all 500 people by name in every letter he wrote; people will not believe if they don’t want to believe.
Some people are too skeptical, so be it. I think Paul could have included something. For example, look at these names from chapter 1 of First Corinthians —
Greetings from Paul - This letter is from Paul, chosen by the will of God to be an apostle of Christ Jesus, and from our brother Sosthenes. I am writing to God’s church in Corinth, to you who have been called by God to be his own holy people. He made you holy by means of Christ Jesus, just as he...
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12 Some of you are saying, “I am a follower of
Paul.” Others are saying, “I follow
Apollos,” or “I follow
Peter,” or “I follow only
Christ.”
13 Has Christ been divided into factions? Was I, Paul, crucified for you? Were any of you baptized in the name of Paul? Of course not!
14 I thank God that I did not baptize any of you except
Crispus and
Gaius,
15 for now no one can say they were baptized in my name.
16 (Oh yes, I also baptized the household of
Stephanas, but I don’t remember baptizing anyone else.)
— NLT
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So, in addition to Paul’s own name and in addition to Christ of course, there are 5 additional names which are listed. But when we get to chapter fifteen and the claim that “more than 500” saw Christ post-Resurrection, not a single name is given. And even more seriously, not a place either.
I’m going to stick with my theory that Paul had heard this, and expected to hear more and have the claim firmed up as time went along. This didn’t happen, and he stopped including it. To his credit. Paul might have been in the state of not knowing one way or the other, regarding this particular claim.
1st Corinthian is the 3rd earliest letter of Paul. He had ample chance to hear more, but he didn’t.
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PS If I was still an evangelical Christian, I’m afraid I’d probably be in the category of “I follow only Christ.” Meaning, I’d be trying to go it alone.