World English Dictionary definition:

Catharsis:

1.(in Aristotelian literary criticism) the purging or purification of the emotions through the evocation of pity and fear, as in tragedy

2.See also abreaction the bringing of repressed ideas or experiences into consciousness, thus relieving tensions

3.
purgation, esp of the bowels

World English Dictionary definition:

abreaction : the release and expression of emotional tension associated with repressed ideas by bringing those ideas into consciousness

Religious catharsis: a sudden emotional breakdown or climax that constitutes overwhelming feelings of great pity, sorrow, laughter, or any extreme change in emotion that results in renewal, restoration, and revitalization.- New World Encyclopedia.

The definitions in and of themselves reveals the cause of catharsis. I imagine that numerous people who saw the movie “The Ten Commandments” starring Charleton Heston  had a catharsis. He was the embodiment of Moses to many.  Many other movies with a religious theme no doubt produced numerous adherents to the Christian faith. In the end, however, it was all a mental change within the mind of the individual with no supernatural event having been involved. Emotions never were  based in reason. Appealing to reason never has been as easy as appealing to emotion.