Note 12 at 1 Cor.15:6: This instance which Paul is referring to is not clearly shown in other scriptures unless this is the same occurrence as recorded in Matthew 28:10 or when Jesus ascended back to the Father (Mk. 16:19; Lk. 24:51; Acts 1:9-12; see note 1 for 1 Cor. 15:6, p. 561).

The use of the phrase "after that" in the next verse means that the Lord's appearance to James and the rest of the apostles came after His appearance to the 500, mentioned here. Unless His appearance to James was after His ascension, it would appear that this showing of Himself to the 500 would more correctly be associated with the Matthew 28:10 instance or some other occurrence not recorded in scripture.

Note 13 at 1 Cor. 15:6: By Paul referring to this large number of people who saw Jesus after His resurrection and mentioning that the majority of them were still alive, he was seeking to verify that Jesus did, in fact, physically rise from the dead. As the number of witnesses increase, the odds of a conspiracy decrease. You might be able to get a small number of people to lie about something and stick to a fabricated story, but that would never happen with a large group of people. Someone would make a mistake or his conscience would force him to tell the truth.

The fact that this was over twenty years after the resurrection further verified their witness. As time passes, memories fade. There is no way that Paul could offer these people as sources to verify the fact of the resurrection if they had just made it up. Their witnesses would certainly contradict each other. Such was not the case. They still held to one account of the resurrection because it truly happened.