Reba said:
Satan is your god if you believe in anything other than the true God.
This is a way to put down other belief systems as false, wrong and dangerous if they are not the same as whatever versions of the Christian belief system includes this statement. Those versions of Christianity may apply this statement to other religions and other versions of Christianity like Gnostic Christianity. It is to prevent loss of people to other belief systems and nonbelief.
Since it is applied to all other belief systems, it is applied to belief systems that don't have Satan and belief systems that are structured differently from Christianity. This sort of Christianity teaches that good will win over evil one day. There are other belief systems that have a balance between opposite forces such as yin and yang that cannot exist without each other. This kind of Christianity holds that Satan is absolutely evil and will be defeated forever in the future. Since yin and yang are not Christian, the kind of Christianity being considered here would claim that believers in yin and yang have Satan as their absolutely evil god who is misleading them. I see that as the same kind of thing as putting words into others' mouths is.
What about people who do not have beliefs? They don't believe in gods and beings like Satan. Saying that if they're not Christian, they have Satan as their god goes against their position of not believing. It's making false claims about other people for the benefit of one's own belief system to scare people away from examining other beliefs and what it would be like to not believe.
The idea of saying that people that don't share one's belief system have Satan as their god is part of the collection of ideas that make up the versions of the Christian belief system being considered here. It's one of the ideas that has an internal maintenance function to keep people within the belief system. It might even be used in attempts to scare people into the belief system. The people with the belief system would have a harder time seeing this idea for what it is because they see things from inside the belief system. It traps the believers into a box by coloring their view of other belief systems and nonbelief.
It is not surprising that there are such beliefs like this one about Satan being the god of disagreers. Belief systems that have characteristics that help them to survive by doing things like holding onto members and collecting new members are the ones that are more likely to survive. Christianity has beliefs such as the Satan one to help it to survive in a world with other belief systems that compete for mindshare.
There are Christians who think of atheism as a religion. It is not one because it is about if there is belief or not in gods, not an entire system of beliefs accepted without question like the various versions of Christianity are. Considering atheism to be a religion seems to make it easier for those Christians to apply the above statement to say that Satan is the god of nonbelievers. Those people are thinking inside the box and project their desire to believe onto everybody else, resulting in them thinking that everybody else has to believe in something. Them being so boxed in doesn't allow them the flexiablity to think about what it would be like to not believe in any of those beings.
Some Christians might say that nonbelievers might think that they don't believe in any such beings when Satan is really tricking them and controlling them. The clincher here is evidence. The Christians had brought forth no evidence that withstands examination. They might claim that faith is enough for such things, but equally valid claims could be made that another god, such as saying that Odin is the True God and that Loki is the god of people who don't agree with that or saying that Amun-Ra is the True God and that Set is the god of disagreers. Without evidence no such claims about gods, Satan or any other such being should be given serious weight because they have equal validity. Claiming that Satan fooled me into making such statements is as valid as me claiming that Loki or Set made the Christians say that Satan is the god of others.