Credulous
Part of speech: adjective
Definition: Excessively ready to believe things; gullible.
Example sentence: The danger to society is not merely that it should believe wrong things, though that is great enough; but that it should become credulous, and lose the habit of testing things and inquiring into them; for then it must sink back into savagery.
Believable
Part of speech: adjective
Definition: Capable of being believed; credible.
Example sentence: The Marxist theory of ideology is extremely contentious, not least because it is tied to socio-economic hypotheses that are no longer believable.