Can't we all just get along?

The Weekly Standard's online editor, Jonathan Last, has a piece on this November's Godless Americans March on Washington (link via Amy Welborn). Speaking of the commonalities which all of the various atheist groups share, Last points out that the first such commonality is a preoccupation with Christianity. Why is that? Christians are no longer the largest religious group in the world (Muslims now have that honor); Christians are hardly the only religious group (including self-proclaimed atheists) to commit atrocities against their fellow man (this being a common charge against Christianity); and atheists secularists have spent bunches of time and energy "proving" that the positives of Western Civ have nothing to do with Christianity. So why the preoccupation? I suppose it might be as simple as the fact that most atheists in this country were raised Christian, but I wonder if there is more to it.

[Update: a sharp reader noted that there are still more Christians than Muslims in the world... I inadvertently compared the total number of Muslims to the number of Catholics, not all Christians. Nonetheless, I'm still not sure that it's the numeric superiority of Christians which gets atheist ire up.]