A history teacher of dubious reputation once told me that "hocus pocus" comes out of the medieval Catholic church, and originated as a corruption of the phrase "Hoc est meum Corpus" ("This is my body") by illiterate priests who couldn't remember the few words of Latin they'd been taught.
Anyone have any feedback on that?

Hocus Pocus does indeed, seem to be from a faux Latin, but of far more recent origin. Both the OED and my 1919 websters agree on a 17th century origin, associated with a juggler, perhaps called Hocus Pocus, who used a fake latin chant to obscure what he was doing and confuse the audience.