Not all disbelievers consider themselves to be atheists, says the Pew Forum on Religion and Public life:
According to the U.S. Religious Landscape Survey, conducted by the Pew Research Center’s Forum on Religion & Public Life, 5% of American adults say they do not believe in God or a universal spirit, but only about a quarter (24%) of these nonbelievers actually call themselves atheists.
Within this 5% of the total population, what other kinds of unbelievers are there? 35% consider themselves “Nothing Particular” and 15% “Agnostic,” while 14% consider themselves Christian, and 10% identify with other religious traditions such as Judaism (4%) and Buddhism (3%).