When talking about Christianity, people inevitably end up talking about "the Church," as if all the denominations were still (if they were ever really) all the very same. But the thing is, despite the fact that all of them talk about this dude named Jesus who said some pretty awesome and radical things about 2000 years ago—and I'm not just using the words "awesome" and "radical" in their 90's slang sense, though those meanings could certainly apply as well—they all have their own different ways of creating and maintaining their communities of faith. Don't believe me? Check out Roman Catholic, Eastern Catholic, Baptist, Lutheran, and Presbyterian churches and attend their worship services, and you'll see precisely what I mean.
In spite of the many similarities between them, these denominations are not all one. If they were, there'd be no need to give them different names. So really, when talking about Christianity, one might more accurately talk about "The Churches" than about "The Church."