In the course of my Orphic studies, I had occasion to look up the origin of the word mystery, and on checking the OED discovered to my surprise (though I think I used to be aware of this, many years ago) that there are two words thus spelled, the usual one (from classical Latin mystērium ‘secret,’ pl. ‘secret rites,’ in post-classical Latin also ‘mystical or religious truth,’ pl. ‘Christian rites,’ from ancient Greek μυστήριον ‘mystery, secret,’ pl. ‘secret rites, implements used in such rites’) and a now obsolete one with the following senses (I give the most recent citation for each):
1. Ministry, office; service, occupation. Obs.
a1533 LD. BERNERS tr. A. de Guevara Golden Bk. M. Aurelius K vii b, None should be taken from the misterie and office that he occupied.
2. a. Craft, art; a trade, profession, calling. Now arch. 1957 Listener 25 July 141/1 We usually start with some sort of prejudice against the verse-writer who is better known as a writer of prose: there is a (very proper) feeling that the two are different mysteries.
b. Skill, expert technique. Obs.
1726 SWIFT Gulliver II. IV. vi. 87 Because I had some Skill in the Faculty, I would.. let him know the whole Mystery and Method by which they proceed.
c. art and mystery n. (also science and mystery and variants) the art and craft of a trade; also in extended use.
1934 A. G. STREET Endless Furrow xv. 254 Talk about old Nicholas Crawford’s art and mystery in grocerin’, why, that’s an open book compared to farmin’.
3. A trade guild or company. Now arch. and hist.
1964 Welsh Hist. Rev. 2 307 The shoemakers, who later formed their own mistery, were already numerous enough in the lordship in 1400.
The etymology says it’s from “post-classical Latin misterium duty, office, service (from 11th cent. in British sources), occupation, trade (from 13th cent. in British sources), guild (from 14th cent. in British sources), altered form of classical Latin ministerium MINISTRY n. by confusion with mystērium MYSTERY n.” and adds “In senses 2 and 3 the word may well have been influenced by or confused with MASTERY n.” I love this sort of confusion, which is so irritating to purists!