The issue was how often in the Bible that the claim of 500 witnesses to the resurrection could be found. Paul was the only one that made that claim as far as I can seen. The website that you used should have worded their claim. For example: "The Bible claims at least 12 times that eyewitnesses saw Jesus after his crucifixion, one claim is that as many as 500 saw him." That would have more accurately represented what the Bible says.
Now I remember. Yes, Paul was the only one who said about 500 people saw Jesus. Given that Paul said he was an apostle of Jesus Christ, that all scripture was given by inspiration of God, that Paul said he did not lie, I accept what he said.
The 1st century accepted Paul to be no less of an authority than Moses. Paul said he was sent by God and that he spoke what God told him to speak. Having some understanding of Paul's gospel, I have no trouble whatsoever seeing how it fits perfectly with the rest of scriptures.
Paul revealed the mystery (1 Cor 2:7, Eph 1:9, Eph 3:3, et. al.) that God had kept to himself until He told Paul. Jesus didn't even know about the mystery ("secret" is a better word). That is why he told his disciples that some of them would live to see his return. The only scriptures Jesus had were Genesis to Malachi. According to them (Daniel specifically) there would be 7 years between his first coming as a lamb led to slaughter and his second coming as King of Kings. He had no idea about God's secret plan to create a new assembly (church) composed of born again believers who were neither Jew nor Gentile. Of course all of that apparently takes some time, 2,000+ years so far. Jesus was correct with the information he had available.
We don't know how much longer this present age will last, but eventually Jesus will return and finish the work God promised Israel. To see God's complete dealing with Israel (and Gentiles who converted), just read Genesis through John, and then go to Revelation. Everything between John and Revelation is God's dealing with the assembly of the body of Christ. Israel is the bride, Christians are the body. The books of Romans through Thessalonians form the Magna Carta of Christianity. They represent the first comma in Isaiah 61:2,
To proclaim the acceptable year of the LORD, and the day of vengeance of our God; to comfort all that mourn;
You might notice that in Luke 4:19 Jesus stopped in the middle of this verse when reading Isaiah 61:2. Why? He could make the claim that he represented the acceptable year of the Lord, but the day of vengeance was yet in the future, i.e. his second coming. Between those two advents we have the mystery (secret), the age of grace in which we currently live.
All of the above is why Paul is so misunderstood and why so many think he is a fraud. But when properly understood, Paul's revelation of the mystery, that God would make a new creation out of both Jew and Gentile, is the epitome of scripture as far as I can tell.
Have you ever heard of that mystery in the church? Maybe, but odds are that you didn't. Despite the fact that it is "what's happening" today, the churches are by and large silent. For now, score one for Satan I guess. But it'll all come out in the wash!
