Generally speaking, I find Theists to be more predictable and thus 'readable' when dealing with them, whereas atheists and fake 'believers' are much more complex, by and large, to predict and avoid being manipulated by.
I would have a Theist do an up-front mission and an atheist do an undercover, long-term, complex mission if I worked for an Intelligence Agency. I would also polygraph them to make sure that I understood how 'true' their Theistic faith vs doubt in god and religion was.
An agnostic, deist or a Pagan Theist may be suitable for the atheist level of complexity and unpredictability.
Theists, who genuinely believe in their faith, are people I appreciate for their honesty and predictability. They are very reliable people who will struggle to lie or alter themselves to suit situations where they must be fake. This is something that I appreciate about them. It has its downside and trade-off though.