Atheist superheroes?

[Sword of Christ]

From the writer's imagination, of course. In a fictional setting the background details can be glossed over unless they're vital to the story somehow. It's like any other fictional, mythological work.

If you want to establish how these "fake angels" aren't the same things as "real angels" you would still need to explain the difference within the setting. Angels who are (or can potentially be) hostile to humans are generally called something else though.