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Just because people are prepared to suffer for their belief systems doesn't mean they have any credibility!
Do have a think before you post, Floo. It clearly had some credibility in their eyes at least. It doesn't thereby mean they were correct to believe it though.
Look at suicide bombers who are more than happy to be killed for their faith!
Yes, it is credible in their eyes. "Credible" means, "Able to be believed; convincing." They are seemingly convinced by it. They are able to believe it.