Now, judging by what I read here, there are those who think it means "sort of vaguely Calvinist" and Unitarians and maybe Episcopalians are not really "Protestant".
And the question of whether the word is a "proper" (as opposed to vulgar) noun (especially when one considers that its form is that of a participle, a verbal adjective, so it must be an adjective used as a substantive -- my head hurts) is , well, confusing.
The target doesn't define itself. So let's just fire a lot of rounds and hope some of them hit. It may not be right, but it's sho' 'nuff understandable.