When I speak of religionists I usually get support from folks who think our faith should be less "institutional" but I'm not really talking about that. Christianity is of course a religion. James 1:26-27 speaks of Christianity as a religion.
However a "religionist" makes his religion his religion. Or plainly, the practice of a person's religion becomes his religion. How he performs. How he worships. How he speaks. It really doesn't matter what religion it is. It could be Christianity, Islam, or some other religion. Everything becomes about the religion. There is nothing else but it. Now, perhaps some should take me to task that devotion to one's faith demands it unless the person be a mediocre or nominal believer. But again, I'm not talking about whether a person is very devoted or not. I'm talking about whether a person can even see anything else but the practice of his religion. Whether we like it or not, historically the world has not been about a specific religion. Sure, we have had large periods of history where a religion dominated ever aspect of life, however even then people's concern was living day-to-day despite that dominance.