There are different levels of initiation. The lower levels are indeed more "atheist-like" but the higher levels definitely believe in and serve negative spiritual entities.
The name, "Illuminati", actually means the opposite of what is says. That is the usual way of these kind of organizations.
So they tie in with Christian Churches? Was Paul anyway involved in creation or control of Illuminati?
Yes, they have subverted and infiltrated Christian Churches. Most notably the Catholic and Mormon Churches.
I don't think Paul was involved in any of this though. Control of the Catholic Church started mainly with the first council of Nicaea.
They are many shades of gray regarding the involvement of the Illuminati, though. Many people serve them unwittingly, half-heartedly, or independently of being formally organized.
One of the tactics of the Illuminati is to continously reorganize and change their "facade" making it hard to pinpoint them. They also infiltrate and misuse legitimate and well-meaning organizations and movements.
The principle is to always work hidden in the shadows behind multiple layers of deception, never actually doing anything much themselves, but instead let lackeys and fools do the work for them.