I don't seem to understand why anybody would make such a correlation. I think the avarage IQ depends on how wealth and how well developed your country is, and not how many believe in religious stuff.
You're looking at it the wrong way. Of course saying the average IQ depends on religious belief is nonsense[1], but the other way around ie. saying the likelyhood of believing in religious stuff depends on how high your IQ is might make sense to some people.
1. Religion tends to suppress and influence education, if that has any effect on the IQ of the population has yet to be proven
1. Religion tends to suppress and influence education, if that has any effect on the IQ of the population has yet to be proven