tmc, there's nothing that cannot be harmful, atheism included. I certainly don't fear or loathe atheism, but there are certain atheists that are as toxic as the worst theists and also those who are as saintly as any religious saint. On balance, I do believe that religion has been a force for good.
Atheism can't literally be harmful, although certainly people who are atheists can. Literally, atheism is "nothing", but ironically a nothing that is something in the sense that it is something that people use to define the lack of belief in religion. It is "nothing" in the sense that it is a lack of belief.
An atheist lacks a belief. Some atheists would say that there is no "Great Spirit" or whatever word for the Divine. A belief in "nothing" is only harmful if that person doesn't have a belief in something.
(eg good, or humanism, or something else that defines their ethics).
I tend to believe that religion has been historically divisive and leads to war, but since there are pacifists who are theists, that is certainly a good thing. Although I could say that in the same way atheism can't make a bad person good, religion can't either. Being ethical is a choice, and beliefs about religion seem irrelevant to that choice. part 1 of 2