My gut tells me that these are words frequently misused, so it's time I set myself straight, and DD has prompted me to do so.
Credulous comes from the Latin "credere" for "to believe", therefore, credulous means "believing easily", so it refers to the perception of a person and not the quality of the thing or intangible. Young children lose their credulous nature as they are exposed to the realities of the world around them. Naivete is merely credulousness in adults. Credulous clients allow Plaintiff's counsel's to convince them about the ongoing merits of their case, even in the face of negative court rulings. But not, the credulous witness or evidence was not believed by the jury. The correct word there is credible, which also comes from the Latin "credere". So the difference here is the suffix. "ous" means "possessing or being full of" the noun while "ible" means "susceptible of or capable of being" of that verb. The part of speech is largely irrelevant and only becomes an issue due to the switched perspective. Therefore, credulous correctly means "possessing or being full of belief" and credible means "capable of being believed". See various v. variable. Credible, therefore, can be used not just with people, but also with tangibles and intangibles, since the quality of capable of being believed is latent in the tangible or intangible until it is acknowledged by the potential believer. One who is credulous may believe too quickly that which appears tacitly credible. Fill in your own subjects; mine are Plaintiff's counsel and their own arguments.