The U.S. Religious Landscape Survey
Here's the most comprehensive poll on religion in the U.S. I could find. Note that freethinkers, at 10.3%, represent more than twice the number of non-Christian religious adherents. The 10.3% number is derived by subtracting the 5.8% "Religious unaffiliated" from the 16.1% total of "Unaffiliated".
Based on interviews with more than 35,000 Americans age 18 and older.
The survey finds that the number of people who say they are unaffiliated with any particular faith today (16.1%) is more than double the number who say they were not affiliated with any particular religion as children. Among Americans ages 18-29, one-in-four say they are not currently affiliated with any particular religion.
People moving into the unaffiliated category outnumber those moving out of the unaffiliated group by more than a three-to-one margin. At the same time, however, a substantial number of people (nearly 4% of the overall adult population) say that as children they were unaffiliated with any particular religion but have since come to identify with a religious group. This means that more than half of people who were unaffiliated with any particular religion as a child now say that they are associated with a religious group. In short, the Landscape Survey shows that the unaffiliated population has grown despite having one of the lowest retention rates of all "religious" groups.
While those Americans who are unaffiliated with any particular religion have seen the greatest growth in numbers as a result of changes in affiliation, Catholicism has experienced the greatest net losses as a result of affiliation changes.
More than six-in-ten Americans age 70 and older (62%) are Protestant but that this number is only about four-in-ten (43%) among Americans ages 18-29. Conversely, young adults ages 18-29 are much more likely than those age 70 and older to say that they are not affiliated with any particular religion (25% vs. 8%). If these generational patterns persist, recent declines in the number of Protestants and growth in the size of the unaffiliated population may continue.
Muslims, roughly two-thirds of whom are immigrants, now account for roughly 0.6% of the U.S. adult population; and Hindus, more than eight-in-ten of whom are foreign born, now account for approximately 0.4% of the population.
Jehovah's Witnesses have the lowest retention rate of any religious tradition. Only 37% of all those who say they were raised as Jehovah's Witnesses still identify themselves as Jehovah's Witnesses.
People not affiliated with any particular religion stand out for their relative youth compared with other religious traditions. Among the unaffiliated, 31% are under age 30 and 71% are under age 50. Comparable numbers for the overall adult population are 20% and 59%, respectively.
Mormons and Muslims are the groups with the largest families; more than one-in-five Mormon adults and 15% of Muslim adults in the U.S. have three or more children living at home.
Among people who are married, nearly four-in-ten (37%) are married to a spouse with a different religious affiliation. (This figure includes Protestants who are married to another Protestant from a different denominational family, such as a Baptist who is married to a Methodist.) Hindus and Mormons are the most likely to be married (78% and 71%, respectively) and to be married to someone of the same religion (90% and 83%, respectively).
Men are significantly more likely than women to claim no religious affiliation. Nearly one-in-five men say they have no formal religious affiliation, compared with roughly 13% of women.