:Other Wikipedia articles should not be used as sources, because this is circular. Granted, WP:CIRCULAR pertains to use of them as sources in other articles, but I think the principle is still somewhat applicable in discussion like this one. Simiarly other wikis should not be used, as per WP:USERG. It is better to use credible sources like Merriam Webster, which defines conspiracy theories as "a theory that explains an event or set of circumstances as the result of a secret plot by usually powerful conspirators". It does not include low credibility as a criterion. Neither does the Wikitionary entry contain third definition you attribute to it. 

It does contain a definition similar to the second one you list here, but if we assume as a given that that definition is correct, how does the Jesus myth idea not conform to it?   

Even if we did use other Wikipedia articles, the Wikipedia article on conspiracy theory does not mention that phrase or that criterion at all. The closest that article comes to the issue of credibility is when its Lead section says that "conspiracy theories are viewed with skepticism", which is sourced.