Check out the t-shirts that were the subject of a recent post at Friendly Atheist. I am not going to get into the subject of children wearing such shirts at school here because I want to focus on a more general issue. Imagine an adult wearing one of these shirts in public. I may not agree with their message (i.e., "Islam is of the devil"), but that is irrelevant. I still support such an exercise of free expression.
Since many atheists have a rather low opinion of Islam, you may say that this particular shirt is an easy one to support. Fine. Imagine that the shirt instead said "Atheism is of the devil," "Atheists are evil," or even "Kill all atheists." I would still support the right of a Christian to wear such a shirt even though I certainly wouldn't like the message.
Here's the catch - the right to free expression goes both ways. I defend the right of the Christian to wear such a shirt, and I expect the Christian to defend my right to wear a "fuck the skull of Jesus" t-shirt (I don't actually have such a shirt). The thing is, I would be surprised if more than a handful of Christians would actually defend my right to wear such a shirt.
At least in the U.S., a Christian wearing a blatantly anti-atheist shirt is going to emerge unscathed. On the other hand, I cannot imagine an atheist wearing a blatantly anti-Christian shirt surviving long here in Mississippi or many other places in the Bible Belt. Even pro-atheist shirts are going to be perceived as vile assaults on Christianity!
This tells me that the playing field is far from level and that Christian privilege is pervasive. Their cars will not be vandalized on account of their Jesus fish, but I cannot display a Darwin fish. Disparaging comments about atheism are commonplace, while criticism of Christianity incurs wrath. Christian billboards are all over the place, but atheist billboards are rarely tolerated. I could go on and on, but I am confident that you get the idea.