When criticizing Christianity, what is it that we criticize? Are we only attacking a "false" form of Christianity, ignoring the "True Christianity" which is immune to such comments? That is certainly what many Christian apologists believe, basing their arguments upon subtle but often question-begging assumptions about what qualifies as "real" Christianity.
This is a difficult but important issue. Naturally we must have some idea of what we mean by "Christianity" when we critique Christian beliefs and doctrines. At the same time, defining Christianity is notoriously difficult. There are all sorts of different groups which claim to be Christian, each teaching different (and sometimes contradictory) things. Christians themselves regularly argue that other self-professed Christians aren't "real" Christians and should not be considered as such.
So where can and do skeptics stand in all of this? We need, I think, to distinguish between Christianity "in general" and Christianity as expressed in particular social, cultural, and historical circumstances. We can discuss certain things about Christianity "in general" - such as what type of religion it is, where it can be found, how it has developed, what some common doctrines are, etc. Such discussion is, however, necessarily limited.
Where much of the real interesting discussion occurs is when we focus upon particular expressions of Christianity - expressions which do not define Christianity entirely but which do qualify as valid forms of that religion. There is no one single Christianity which is the "true" Christianity to which all denominations strive, except perhaps as an ideal. And not all Christians would agree on what that ideal should be, if one exists.
The different forms of Christianity are dependent upon particular circumstances which might include social or cultural factors (such as race, economics and class, technology, etc.), political factors (ideologies, philosophies, etc.), and historical factors (nationality, ethnicity, etc.). All of these combine in a variety of ways, often unseen, to influence how Christian beliefs develop, are interpreted, and are used by individual Christians in their lives.
The fact that history, economics, and politics can play an important role in religious beliefs is something often noticed by skeptics, but sometimes denied by religious believers - all too often, believers insist that their doctrines are "pure" or "holy," untainted by the profane world around them. At the same time, though, some skeptics forget the interaction of society and religion as well, thinking that a critique of some particular expression of Christianity also qualifies as a critique of Christianity generally; sometimes this may indeed be true, but usually it isn't.
What this means in practice is that any critique of Christianity is best when carefully qualified: it is a criticism of a particular form of Christianity that arises in a particular place and time among a particular people. This critique might not apply to Christianity generally, although there is at least an implicit critique of the fact that whatever Christianity might be, it isn't the sort of religion which easily prevents some odious version of itself from developing. Considering how many Christians argue that theirs is the True Religion, that isn't a mild criticism.
