People unaffiliated with any religion constitute nearly 20 percent of the American public, making them almost as numerous as Catholics, who accounted for 22 percent of the participants in a recent Pew Research Center study. The survey, released on Oct. 9, found that people who say they are atheist, agnostic or “nothing in particular” grew by almost 5 percentage points since 2007, from 15.3 percent of the population to 19.6 percent. The greatest shift toward “nothing in particular” apparently came from Protestants, who now make up 48 percent of the population, compared to 53 percent in 2007. “These are not necessarily nonbelievers,” said Greg Smith, senior researcher for the Pew Forum on Religion & Public Life. “They’re just not associated with any faith in particular.” The unaffiliated tend to be younger than the general public, the survey showed, with 35 percent between ages 18 and 29.