If it seems most everyone in your church used to be "something else," they did. More than 33 million adults in the United States reported they had changed their religious identification or preference at some point in their lives, according to a study by the City University of New York. That's 16 percent of the total adult population. Demographers call this faith-shifting phenomenon "religious mobility," and it's on the rise.

The U.S. population remains predominantly Christian. In 2001, 77 percent self-identified as Christian, although that number is down from 86 percent in 1990. The number of people identifying with any religion also declined in the same period, from 90 percent to 81 percent.

While mainline Protestant groups are declining, evangelical and charismatic denominations are on the rise. Of those who identify themselves as "evangelical/born-again," 37 percent "switched" to the faith from another religion.

Who's moving: The shift is greatest among mainliners, smaller faiths and ...