Its not theism in itself. And I would argue its not rationality in itsef for a sufficient value of well... values.

Compare the fate of the very pious and irrational Amish (a table of who's demographic success I present in another comment) to the very pious and irrational shakers.

Membership in the Shakers dwindled in the late 19th century for several reasons: people were attracted to cities and away from the farms; Shaker products could not compete with mass-produced products that became available at a much lower cost; and Shakers could not have children, so adoption was a major source of new members. This continued until the states gained control of adoption homes. Some Shaker settlements, such as Pleasant Hill community in Kentucky, and Canterbury, New Hampshire, the latter of which died with its last member, Ethel Hudson, in September 1992,[6] have become museums.

Although there were six thousand believers at the peak of the Shaker movement, there were only twelve Shaker communities left by 1920. In the United States there is one remaining active Shaker community, at Sabbathday Lake, Maine, which as of September 2010 has only three members left, Sister June Carpenter, Brother Arnold Hadd, and Sister Frances Carr [7] [3][8]. The Sabbathday Lake community still accepts new recruits, as it has since its founding. Shakers are no longer allowed to adopt orphan children after new laws were passed in 1960 denying control of adoption to religious groups, but adults who wish to embrace Shaker life were welcome. This community, founded in 1782, was one of the smaller and more isolated Shaker communities during the sect's heyday. They farm and practice a variety of handicrafts; a Shaker Museum, and Sunday services[9] are open to visitors. Mother Ann Day is celebrated on the first Sunday of August. The people sing and dance and a Mother Ann cake is presented.

As a note of interest let me just point out that both groups where for a extended period of time of a size comparable to the number of regular LW readers.