I take it that few of us would claim anything much about what all Christians think, or even what the overwhelming majority of Christians thinks about particular political or moral topics (other than perhaps at the most general level, or about topics on which there’s very broad agreement among Christians and non-Christians alike).
The categorical generalization about what all or even nearly all Christians believe is clearly unsound. The statistical arguments may theoretically be accurate, but in practice this data is hard to collect: Our sense of what most people of group X think is often based on highly unrepresentative samples (e.g., those Xs we know, or those who are especially loud, or even those who live in our country rather than elsewhere). It also often does not distinguish between very different flavors of a political belief and intensities of that belief — for instance, knowing how many members of some group “oppose abortion” tells us rather little about their specific policy views, or the intensity of those views.
You might think that knowing someone’s religion, at the coarse level of Christianity, Islam, and the like, would indeed tell you a good deal about his actual moral beliefs. But it turns out not to be so. It’s not so for Christianity, and as best I can tell it’s equally not so for Islam (or even for smaller religions such as Judaism). There may be a few exceptions; I can confidently guess, for instance, that most Christians oppose laws banning Christianity. But those really are exceptions.
As a result, it makes little sense to say, for instance, “Christianity is a religion of peace” or “Christianity is a religion of war,” unless one is speaking about theoretical aspirations about what the religion should be, rather than describing how Christians actually act, have acted, and are likely to act. It seems to me the same is true for Islam.
I should stress that I think it’s perfectly proper for people to criticize religious beliefs and religious movements, just as it’s proper for people to criticize any ideology. But broad criticisms about how Islam is this or Christianity is that — again, with a very few exceptions — are almost always based on inaccurate overgeneralizations.