Sometimes one hears theists make the statement that "everyone believes in god". Those who say this kind of thing will maintain that this is true even if you stand right in front of them and tell them you think god is a fairy tale. Obviously, the only way they can think this is to believe that everyone somehow "knows" that god exists, but some people perversely deny it for emotional reasons.
Any theist who says this sort of thing has a closed mind and is almost certainly fanatical. There is little hope of reasoning with such a person at all. On the other hand, the position is so extreme and intolerant that it lends itself to easy ridicule and attack. One of your best quick and simple replies is to say:
"You think everyone has to be just like you?"
This response is probably best if you don't want to get into any sort of debate with such a person, which is probably a good idea. Point out the inherent idiocy at the core of the statement and then drop the subject.
This attitude is a variation of the "no atheists in foxholes" argument--but with a twist. Both are based on the assumption that a person's emotions completely determine what he or she believes. The "no atheists in foxholes" argument is an implicit admission that religion is a delusion. This version is an implicit admission of how theists think: They "feel" and then rationalize their feelings, therefore they are quite sure that everyone else does the same.
While it is true that everyone's thinking is effected by emotion, the extent of this effect will vary from individual to individual just like any other trait. Many people, especially non-believers, know this and therefore actively try to prevent their emotions from having primacy in their reasoning. They know that allowing emotions to determine their reasoning can interfere with the accuracy of their thinking. For a theist, however, their emotions always have primacy in their reasoning, and they apparently can't imagine that anyone could be different. (They even think that this is a good thing because they can't distinguish between emotions and values--but that will be the subject of another post.)
One of the things you can say is to first make this explicit:
"You think atheists are just in denial? Why are they in denial? They just don't want to admit it for emotional reasons?"
Once you get an explicit admission of the assumption that everyone is controlled by their emotions, you can ask:
"You think everyone's thinking is controlled by their emotions?"
As an argument for the existence of god, this is an appeal to "groupthink"--the idea that truth is determined solely by what everyone believes to be true. This is very popular with believers. I have dealt with it elsewhere. It is also an implicit insult to the intellect, maturity, and integrity of non-believers. Thus, you can add:
"To say that no one can think objectively is an insult to the intellectual integrity of the entire human race."
Like most of the "groupthink" statements made by believers, it is also an indication of their insecurity and need for reinforcement of their beliefs by others. They need that reinforcement so much that they will simply imagine that others believe the same as they do if they have to. Thus, at the appropriate juncture, you can say something like:
"You have told me that your thinking is controlled by your emotions. Therefore I can only conclude that you have an emotional need to believe that everyone secretly agrees with you."
"I think you have an emotional need to believe that, just like you have an emotional need to believe in god."
Finally, this sort of "thinking" also shows a distinct lack of empathy because one can't think that this is true unless one is unable to see that others have different inherent personalities, characteristics, and formative experiences. Therefore, like many "groupthink" notions, this "thinking" is a precursor of intolerance--especially in light of the implied rejection of the notion that anyone can actually think differently. If you want, you can add:
"I guess no one is allowed to even think differently from you."
"So, the way you see it, you are always right and people who don't agree with you are just liars."