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Credible or credulous: If you’re credible, you are authoritative, believable. If, on the other hand, you are credulous, then you could be described as gullible or naive.
‘Bill Gates would make a credible judge in any contest between developers of computer software for mass use.’
‘Most religious people are seen as credulous by atheists, as they understand them to be prone to believe in events lacking evidence to back them up.’