It's not Christianity that is bad, it's the people that do bad things that claim to be Christians in this world, like the ones in Africa burning people alive for being accused of witch craft....There is no official religion of the United States, everyone knows that.
However everyone also knows that the dominant religion is and has always been Christianity, and that's why American liberals have and will always attack this particular religion.
When your starting point is the belief that America is the main source of the problems in the world, and when you don't even believe in the foundations of America and it's legitimacy, then naturally you go after the dominant culture.
If Judaism was the largest religion, then Jews and Judaism would be the ones liberals hate the most and the ones who they would claim were just as evil as todays Islamic extremists. Same for any other religion if it were the dominant religion in America.
In this case however, liberals are forced to go back many centuries and way before the history of the United States to find a time when the dominant religion of the day, was filled with atrocities.
Islamic extremists are committing atrocities TODAY, but in the minds of American liberals you must not acknowledge that, unless you at least qualify that acknowledgement by first making the claim that Christianity is just as bad.