This is from the Catholic Encyclopedia entry for Heresy:
"Heresy differs from apostasy. The apostate a fide abandons wholly the faith of Christ either by embracing Judaism, Islamism, Paganism, or simply by falling into naturalism and complete neglect of religion; the heretic always retains faith in Christ."(It is worth noting, at least parenthetically, that making "faith in Christ" a defining feature of "heresy", as opposed to apostasy, is a case of logic that is not so much circular as it is fractal. For if two people have wildly different views of Christ, then in what sense do they both have faith in the same Christ?)
One fascinating side-effect of this definitional shift, is that the Catholic Encyclopedia's entry on Albigensianism is forced to state the following (in order to be consistent with the article on Heresy):
"Properly speaking, Albigensianism was not a Christian heresy but an extra-Christian religion."The implications are truly remarkable. Catharism-cum-Albigensianism is generally (if not universally) considered by historians and religion scholars as the single most important (if not defining) example of medieval heresy!