What is Christianity? That's a difficult question to answer, but it is also an important question. There are obvious implications for Christians themselves: unless they have some sort of definition in mind, how can they discern who is and is not an adherent of their religious faith? But it is also crucial for those who would offer critiques of Christianity because without some sort of definition in mind, how can they tell what and whom they are criticizing?
A very common rejoinder to criticisms of Christianity (or, more often, the actions of Christians) is the idea that we aren't talking about "True Christianity" or "True Christians." That then leads to a discussion about what "Christian" truly means and whether the groups in question fit some particular description. There is, however, a hidden premise in that which needs to be challenged: that there is "One True Meaning" of Christianity out there, independent of us, our beliefs, and our actions.
I don't accept that premise. Christianity is best defined by what Christians do. Thus, Christianity is loving and good insofar as Christians are loving and good; Christianity is brutal and evil insofar as Christians are brutal and evil. That, however, begs the question of just who these "Christians" are.
Who are these Christians? Unless we can identify some independent notion of "Christian" that rises above all cultural and historical contexts, then we must be content with allowing people to define "Christian" for themselves - and that means that whoever claims to be a Christian should probably be accepted as a Christian.
The most reasonable limit on this would seem to me to be that being a "Christian" should involve some belief in or allegiance to "Christ" (otherwise the word itself wouldn't make much sense). Beyond that, I employ a very inclusivist definition of Christian according to which anyone who sincerely and devoutly considers him- or herself a Christian is, as far as I am concerned, a Christian.
I am not in any position and have no interest in trying to convince someone that they aren't really a "True Christian" (tm). That is ultimately a pointless and silly debate that I leave to some Christians themselves as they try to define each other out of existence - an argument that I find alternately amusing and depressing.
Sometimes we may hear that that we should take a look at what the term originally was meant to mean on the idea that this meaning has been corrupted over time. This suggestion contains three critical and questionable premises, each building upon the other:
1. There was a single original meaning.
2. That single meaning can be reliably identified.
3. People today are bound to adhere to that meaning or fall outside the label.
I don't think that we have very good reasons for uncritically accepting any of them - and, if we don't accept them, then the prospect of comparing contemporary uses of "Christian" with an original meaning is pointless in the context of the debate over what constitutes True Christianity.
The simple fact of the matter is, "Christian" is defined in different ways by different groups - and each group has just as much right to use that label as any other. The fact that some groups have beliefs that we find appealing and moral while others don't is irrelevant: the idea that those groups with unpleasant or nasty beliefs can somehow be excluded from the concept "Christian" is simply a form of special pleading known as the "No True Scotsman" fallacy.
The fact that it means one thing to the Roman Catholic Church and another thing to the Pentecostal Churches does not allow us to say that there is some third and independent definition which we can use and thereby determine, objectively and definitively, who is and who is not a Christian. We can tell who is a "Roman Catholic-type Christian" and who is a "Pentecostal-type Christian" by using the definitions created by those organizations, and that is entirely legitimate. But there is no use in trying to step outside of the human context and find some True Christianity that solves our semantic conundrum.
Now, if a group is very unlike most Christian groups, we are justified in considering it fringe Christian group; yet we must remember here that the fringe/mainstream distinction is created solely by "majority vote" and not by some pure concept of Christianity which we are using as an operational standard. If the "majority" of Christian groups change (as they most certainly have in the past), then location of the "fringe" will change as well. At one time, it was "fringe" Christianity to oppose slavery; today, just the opposite is true. At one time, it was "fringe" Christianity to oppose capital punishment; the opposite is not quite true today, but Christianity may be headed in that direction.