This graph might tempt you to believe lower IQ implies higher religiosity. In reality it's much more likely that poverty leads to both lower IQ [1] and higher religiosity [2].
You can't discus religiosity without also discussing the local culture.
In my country (Romania, claimed IQ average in the article being 94), because of the local culture and even though I'm sure there are plenty of atheists and agnostics around, people that are not religious do not usually admit it, because that brings with it the disapproval of society at large. And the local culture does shape the society you live in, being a powerful force against misfits.
In fact, based on my own observations, I think that religiosity is also directly linked with poverty. Due to the economic recession, the orthodox church here has never been more popular in the last 100 years, or more wealthy.
Btw, I am religious, while my IQ is well over average.