The following table shows IQ estimates for the eight religious classifications that had a sample size of at least 40. The IQ scores are derived by setting the white mean wordsum score to correspond to an IQ of 100 with a standard deviation of 15. To avoid language fluency issues, only respondents born in the US are included:
Not quite the one standard deviation advantage that Ashkenazi Jews are elsewhere found to have over white gentiles, but some of these Jews are Sephardic and other ancestrally Jewish but irreligious respondents presumably identified themselves as having no religion.
The high Buddhist score is not simply the result of well-off Asian practitioners. It is, as Christian Lander has identified, a favorite for "spiritual" SWPLs who recoil at the idea of adopting an occidental belief system. More than half of the self-described Buddhists are white.
Like we see in the case of conservatives and liberals versus moderates, non-denominational Christians score lower than Protestants or Catholics. Those who are more intellectually committed tend to be more intelligent than those who opt for the undefined, uncontroversial label.
* Interdenominationalism is a Christian/Jewish hybrid, similar to (although not necessarily synonymous with) messianic Judaism.
GSS variables used: RELIG, BORN(1), WORDSUM