Given the relatively small numbers of atheists in America, it's curious that they would be portrayed as such an incredible threat. Even if we take into account the larger numbers of irreligious, secular, and atheistic people in other Western countries, they still don't seem to be big enough to constitute any sort of menace. Then there is the fact that in countries where atheists are a small minority, they are an even smaller minority in prisons; countries which are more generally irreligious and secular have lower rates of violent crime than countries with higher rates of religious theism.
So what's going on? Where's the big threat being posed by irreligious atheists? There are a couple of things which Christian Nationalists have in mind. Many are worried that atheists undermine the allegedly biblical basis for American law, even though no such basis can be identified. Others worry that atheists will persecute Christians in the same way that Christian Nationalists attack non-Christians. More than a few express concern over communism -- something that might have been half-way reasonable 10 or 20 years ago, but it's persistence today reveals a lack of imagination on the part of Christians who keep bringing it up.
What might be more serious is something that is generally left unsaid: atheists represent a specter of doubt, questioning, skepticism, criticism, and even blasphemy. Irreligious atheists are like metaphysical anarchists who do not submit to the authority of any religious institution, not even those of "false" religions, and thus feel free to criticize all religions. Irreligious atheists call into question the validity of religion generally just by the fact of their very existence. By living, and worse yet by living well, they demonstrate the irrelevancy of religion to having a good life. Christians Nationalists don't understand how they do it, but they know that it's intolerable.
Irreligious atheists are a threat to the leaders of Christian Nationalism for the simple reason that their existence demonstrates that religious leaders just aren't needed. Worse than any criticism is to be laughed at, and worse still is to be dismissed as irrelevant, unneeded, and unimportant. At least when people are laughing at you they are taking you seriously enough to make jokes about you; when you are completely irrelevant, however, you are ignored.
This image is based on a World War II poster warning people of "The Killer" called "Accidents."