Religious landscape survey

For various reasons I had occasion to look up religious affiliation figures for the U.S. I happened upon The Pew Forum's U.S. Religious Landscape Survey, which is the only such survey with which I'm familiar. If you're curious about the composition (by religion) of arguably the most religiously-minded industrialized society in the world, this is a gold mine.

A couple of fascinating tidbits:

• Buddhists in the U.S. slightly outnumber Muslims.
• Mormons outnumber Buddhists and Muslims combined.

Here is perhaps the biggest surprise to me:

The survey finds that constant movement characterizes the American religious marketplace, as every major religious group is simultaneously gaining and losing adherents. Those that are growing as a result of religious change are simply gaining new members at a faster rate than they are losing members. Conversely, those that are declining in number because of religious change simply are not attracting enough new members to offset the number of adherents who are leaving those particular faiths.

I know that some religions require that a nonbeliever marrying an adherent must adopt that religion, but surely that doesn't explain all the comings and goings. I always thought that faith was a core part of one's identity, but evidently that is not always the case.