Suppose the allegations that all three were atheists or had atheistic leanings for some of most of their adult lives are accurate how did that philosophy (atheism) contribute to the mass murders of ethnic and religious groups, the suppression of child bearing by the government and the attempts to eradicate certain deviants and defectives (so-call by them) from the gene pool? Conservative estimates support a death toll of 26,000,000 million between Hitler and Stalin alone.
Atheism isn't a philosophy. It's an opinion. On one thing. When asked, "Do you believe in God?", atheists say, "No". That's all there is to atheism. To call atheism a philosophy is like trying to claim that people that don't drive is a philosophy. Some people don't drive because of well thought out philosophical reasons. Some don't drive because they don't like it. Some don't drive because they're not allowed to. Some don't drive because they've never had the opportunity. There's a huge spectrum of reasons why people don't drive, and they don't have any philosophical, rational, or religious underpinnings in common. The same is true of atheists.
Christians on the other hand have all kinds of commonalities. They all believe that at least parts of the Bible are literally true. They believe that the book was divinely inspired. They believe that salvation can only come through Christ. They believe that the Bible has relevance in today's world, and it's teachings should be studied. So Christians as a whole have a lot more commonality than atheists. One is a group that has a single trait in common, that has no more in common than a group of people that have naturally curly hair. The other is a group that has lots of commonalities, akin to fruit in a world that has lots of fruit and a lot of things that are not fruit.
To compare the "group actions" of Christians to atheists is to draw a false equivalence. One is a group that has many actual, real things in common, while the other is a group that has nothing in common except for a single trait. Actually, it's the lack
of a single trait that the atheists share. A better analogy would be to compare Christians to fruit, with each different sort of Christian religion being a different sort of fruit; Catholics are apples, Methodists are pears, Baptists are grapes, Mormons are bananas, etc. Meanwhile atheists are something other than fruit; secular humanists are trees, atheist Buddhists are eyebrows, nihilists are stereo speakers, etc.
There's another clue here too: it's proper English to capitalize the names of religions, such as Methodist, Mormon, Catholic, or Christian, as they are proper nouns, a name of a religion. One does not capitalize atheist, except at the beginning of a sentence, since it isn't a proper noun. It doesn't denote a religion, philosophy, a group with common traits, or a community of similar people. It only denotes a group that for many, many different reasons doesn't
believe in God, and doesn't otherwise speak of their cultural, philosophical, or religious similarities at all.