Most liberals are Christians if you look at the stats. Since about 86% of America is Christian and it's obvious that not all of that 14% is liberal (KEmperor), it would obviously stand that most liberals are Christian.
Also, the Bible is believed in by tons of people, earning itself the title of the mainstream religion in the US. Meaning it should be protected in ways that a Pagan religion or some cook who calls himself god should not be.
there's a little thing called the First Amendment that doesn't agree.