“Sacred” is an adjective; “the Sacred” is a noun. In The Ideology of Religious Studies, Tim Fitzgerald discusses the adjectival use:
If by ‘sacred’ we mean those things, ideas, places, people, stories, procedures and principles that empirical groups of people value, deem to be constitutive of their collective identity, or will defend to the death, then it seems likely that we have a relatively meaningful crosscultural concept. (19)
By contrast, the noun apparently refers to a non-empirical thing, perhaps an “ontologically distinct transcendental entity or supernatural realm” (19), whose existence must be intuited by its appearances or manifestations. Careful scholarly use of the adjectival form of the term points not to “the Sacred,” nor primarily to sacred things, but rather to humans who hold things to be sacred; the noun use of the term points to something that can’t be seen.
It is advisable not to confuse them, and I take it as self-evident that one of these has more analytic usefulness than the other.
(This post is reblogged from The Bulletin for the Study of Religion)