Episcopacy

Episcopacy

I am reading a book entitled "The Big Test" about the origins of the SAT. The author uses the word "Episcopacy" to refer not to a group of bishops, but to a group of Episcopalians. This seems like very bad English. How can "episcopacy" refer to a group of Episcopalians when it's missing the crucial "al" that transforms the word for bishops into the word for the Church that makes use of them?