Ugh, this old argument. Yes, government enforced Christianity (the vast majority of Christianity, despite what the US government will claim about the seperation of Church and State) has caused a lot of problems and has often been an obstacle to truly free thinking. These other answerers are correct about that.
However, this smug sense of superiority that atheists get because they think are somehow more advanced than those who are religious is bullshit.
Christian thought, when interpreted by those with more open, tolerant minds, has made a significant contribution to anarchist thought, before the atheism of the 19th century came into vogue. Many of the most successful attempted utopian anarchist and communist communities of the past, like those of the Hutterites and the Quakers and even the Amish were founded on Christian principles. "Leo Tolstoy," the genius Russian author was also a "Christian Anarchist."
Hegel and his followers, the Hegelians, the people who influenced Proudhon and Marx to an extreme degree were also Christians, and their original philosophy of dialectical change was based on the interaction between the spirit of God and human society. Proudhon and Marx essentially took that philosophy and replaced God with material conditions, making the theory much more practical. Though Proudhon and Marx, both militant atheists, improved Hegel's theory a lot, it can't be forgotten that they owed their foundations to Christian thinkers.
Though Christianity does affect a person's worldview in some way (depending on that person's interpretation of Christianity), as does any other belief, it is NOT possible to erase a person's culture, no matter how much you disagree with them. The Communists learned that the hard way through their failed "Cultural Revolutions" that attempted to rid people of religion and culture through the use of atrocity and mass murder.
Christianity and anarchism CAN co-exist, as can any other religion and anarchism. Any aspect of culture that doesn't infringe on the rights of others (and no, infringing on the rights of others is not a core tenant of Christianity, that's just an excuse used by those who want to maintain their own secular power) should be allowed to freely exist in any anarchist society. If it can't, then we might as well hang up our hats and give up, because it is not possible to destroy culture, and even if it was, that would be the opposite of anarchy.
I'm not a Christian, so don't take this as an apology, but anarchy relies on tolerance for ALL beliefs and ways of life, and militant atheism does not meet that criteria.