Here's some United States domestic data on percentage of college graduates by religious tradition. Whether or not somebody has graduated from college isn't the same variable as intelligence, but they are probably correlated to some extent. The numbers are from p.16 of the full pdf version of the ARIS 2008 survey.
http://www.americanreligionsurvey-aris.org/reports/highlights.html
In 2008, 27% of the American over-age-25 population were college/university graduates.
Here are some figures for some of the religious traditions, from lowest to highest, with comments by me...
13% Pentecostal/Chrismatic - My impression is that many of these people are drawn from poorer socio-economic groups, especially in the rural South. The wealth variable seems to have a significant correlation with univerity graduation rates. While these people are less likely to have graduated from college, I'd guess that they are above average in their familiarity with the Bible.
16% Baptist - Observations as above, but a little less extreme. This is another group that's apt to emphasize Bible study and it's these groups that we typically find populating the Bible colleges.
25% Roman Catholic - This is close to the national average, as one might expect for a religious grouping this large. (25% of the US adult population self-identify as Catholics.) My impression is that there's lots of variation in this group, ranging from recent arrivals from Mexico with very low education levels, to established Irish and Italian Catholics who probably resemble the Mainline Protestants and come in significantly above the national average.
26% 'Christian generic' - This is one of the fastest growing segments. It consists of people who tell pollsters that they are Christians, but don't belong to any formal denomination. They probably represent a pretty broad spectrum, ranging from fundamentalist members of independent store-front churches to a huge number of secularized people who never attend church but still think of themselves as 'Christian', if only for vague cultural reasons.
27% National Average
31% 'Nones' - These are the people who tell pollsters that they don't have any religious identification.
35% Mainline Protestant - These are the Episcopalians, Methodists, Presbyterians and similar groups like the Lutherans. These are mostly the historical Northern European established churches and these people represent the old-stock Anglo settler population, some of them with roots dating back to Revolutionary War times. They tend to be middle class or above, financially. These churches have been shrinking, loosing adherents at the low-end to the Baptist and similar groups, and on the high-end to the Nones.
57% Jewish - This smaller group has traditionally emphasized Torah and Talmudic study as a religious duty. So study is part of their culture and when the Jews became secularized, that broadened out to higher education in general.
59% Eastern Religions - This is another small group and its largest component is the country's million plus Buddhists. These are divided between Asian immigrant populations and white American converts. The Asians resemble the Jews in their emphasis on pushing their kids into higher education, while the converts are typically college educated. Another thing we've been seeing is many young Asian students coming to the US to study in our universities, then staying here when they graduate and not returning home.
