I have never heard this before. For me, credibility has to do with trust, not beliefs.

It's just that the people who believe a superstition think that only others with the same superstition are credible.

Is that generally assumed? - Mr X believes that it's unlucky to walk under ladders - does the world really find Mr X more credible than Mr Y who happily walks under ladders? Or does it just accept them? - Mr X believes in divine intervention from a Christian god, whereas Mr Y doesn't. Is Mr X more credible in the eyes of the world? - Mr X believes that there are little things called quarks which are the building blocks of matter - he's never seen one and only read about them in books. Is he more credible than Mr Y who thinks all of life is imaginary, the principle of Maya? - Mr X believes in Cartesian dualism and that his mind is separate from his body. Mr Y believes that "mind" doesn't exist, only matter and that we are essentially robots. Mr Z, on the other hand (yes that makes three), thinks everything is "mind". Who has more credibility here? If we take "superstion" to be an "unreasonable notion", then lots of things appear unreasonable (wave/particle duality is another one), and could therefore be described as superstitious. Physicists believed in the aether for a long time - with no "proof" - Michelson & Morley then rubbished the idea. Wasn't the aether a superstition? (Yes, they thought it reasonable - but on the whole physicists now say it was unreasonable.) The Big Bang is a creation myth and can never be proven - evidence can point in that direction, yes, but evidence pointed Ptolomey and his followers to believe in a geocentric universe. What am I trying to say? Mmmm.... probably that virtually everything can be described from some angle/time as unreasonable (life *is* unreasonable, hence why we have a website called "answerbag") and therefore superstitious. All we really have are our beliefs and whether we agree with others beliefs. So I'd think the question should really be: "Whose superstitions do you find credible?" *********************** However, I do see what you're getting at - you're suggesting that religious people are considered more credible than atheistic people. Not where I come from.