The simple fact is that in America, Christianity as both dominant and domineering.
It’s dominant in that more than three quarters of the American population claims to be some variety of Christian. More than nine tenths of our elected officials do the same, with many of them openly pandering to Christian constituents. Even the rapidly growing category of “nones,” (people who claim no religious affiliation) mostly come from Christian backgrounds, and what vague residual spirituality they may retain is informed largely by a Christian outlook. Even most of us who firmly identify as atheists tend to come from Christian families. Christianity pervades Western literature, art, and culture. You can’t drive two blocks in most towns in America without passing the church of some Christian denomination.
This means that atheists in America, aside from encountering Christianity on a daily basis and therefore having more opportunity to argue with its adherents, are also likely to simply be more knowledgeable about it than other religions. I, personally, don’t feel I’m really in a position to make detailed criticisms of Islam, or Hinduism, or Jainism, or Sikhism, or Shintoism, or Buddhism, or any of the nigh uncountable other religions that populate the world. I don’t believe any of them, either, but I just don’t know enough about them to have the kind of detailed conversation that I might have about Christianity. So it would be awful presumptuous of me to write as if I could.
But Christianity is not merely dominant in America, it is domineering. Not content to merely be the dominant cultural touchstone of the nation, agents of Christianity are constantly seeking to control the institutions of society for the explicit purpose of promoting their religion and compelling those who don’t share their belief to follow its behavioral dictates anyway. If someone in America is trying to use public schools to proselytize to your kids, that person is almost certainly Christian. Trying to replace science with sectarian theology in public schools? Christian. Passing legislation to mark government property with explicit totems of their mythology, like a dog peeing on a tree to mark its territory? Christian. Compelling people with business before the government to sit through sectarian religious practices before they can be heard? Christian. Passing laws that compel government to interfere in the most intimate relationships of people’s lives? Christian. Demanding that legislators pretend climate change isn’t real because the science predicts increased floods and “my god promised not to destroy the word with a flood again,” or to ignore it because “my god’s gonna end the world before it matters anyway?” Christian. Christianity is simply the greatest threat to religious pluralism, secular government, and scientific literacy in America. Nothing else is even close.
That’s not to say that there aren’t people of other religions that might support any or all of these things, or other equally ridiculous agendas, if they had the power. But in America, they simply don’t have that ability. Christianity is the only religious interest in America with the numbers and the power to do these things with any kind of large scale success. And so Christianity necessarily gets the most pushback from atheists in America. It’s simply a practical reality.
It’s also not to say that all, or even most, Christians pursue such agendas. It’s just that the vast majority of the people that do pursue them, and virtually all of them that gain any widespread traction, are Christian.
By the way, did you notice how I keep throwing the words “in America,” in there?
In other parts of the world, where other religions dominate, those other religions are the ones that come in for the most criticism from atheists. On occasion I’ll read the blogs of atheists native to places like Iran or Egypt, and I can promise you that those atheists are not spending most of their time criticizing Christianity. They’re “picking on” Islam. Because it’s the religion they grew up with, the religion they know, and the religion that is negatively impacting their lives the most. Everything I said about Christianity in America is true for Islam in those countries, and then ramped up an order of magnitude by the fact that many of them are explicit theocracies.
And I’ll come right out and say it: those people are much braver than I am. You think I’m passionate about what I write? I don’t know that I’d have the balls to do it at all if I lived in a theocracy that gives explicit legal sanction to killing me for it. These people do, and my hat’s off to them.
Of course, it should go without saying that we don’t believe in any other religions either. And that if laws were going to be passed justified solely on the religious precepts of Islam or Native American shamanism, or if our kids’ public school teachers and coaches were setting aside class time to pray to Cybele or Ra, or if our legislators were trying to mark government buildings with “Ph'nglui mglw'nafh Cthulhu R'lyeh wgah'nagl fhtagn,” we’d be opposing those things too. But most of the time it can go without saying because none of those things are a danger in this country.
For that matter, if atheist public school teachers were taking up class time to tell students that their gods don’t exist, I’d be opposed to that as well.
This is kind of similar to the reasoning on why atheists speak out about religion at all. It’s because it affects our lives. Not because any of the religions seem to be true or special, but because of the actions that their followers take in society on behalf of those beliefs. American atheists speak out most about Christianity because Christians are the actors that most affect our lives. Atheists in Muslim countries speak about Islam because Moslems are the actors that affect their lives. It has nothing to do with “picking on” a religion; it’s simply the cultural and geographic realities we have to deal with day-to-day.
That’s really all there is to it.