By Dan Merica, CNN

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Washington (CNN) – After years of marked growth, the size of Americans who identify with no religion slowed in 2012, according to a study released Thursday.

Since 2008, the percentage of Americans who identify as religious "nones" has grown from 14.6% to 17.8% in 2012, according to the Gallup survey. That number, which grew nearly one percentage point every year from 2008 to 2011, grew only 0.3% last year – from 17.5% in 2011 to 17.8% in 2012 – making it the smallest increase over the past five years.

This study contrasts with headlines from previous studies on religious “nones,” including a 2012 study by the Pew Forum on Religion and Public Life that found the group was the fastest growing "religious" group in America and that one in five Americans now identify with no religion.

“Although this ‘rise of the nones’ has increased dramatically over recent decades, the rate of increase slowed last year, suggesting the possibility that there may be a leveling off in this measure in the years ahead,” reports the Gallup study, which is made up of more than 350,000 interviews.

Frank Newport, editor-in-chief of Gallup, says these results suggest “that religion may be maintaining itself or even increasing in the years ahead.”

“Our current ability to look at it over five years with these big surveys suggests the possibility that the growth [of the nones] may not be inexorable,” Newport says.

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In his book, “God is Alive and Well: The Future of Religion in America,” Newport argues that a number of factors, including baby boomers reaching senior ages, migration to more religious states, recognition of health and well being of religion and an increase in a Hispanic population, are all reasons that “we are going to continue to have a quite religious nation going forward.”

Atheist and humanist activists disagree and pushed back against the Gallup study.

“The truth is, it doesn't really matter whether one of these surveys – even a big one like Gallup – shows the number leveling off a bit this past year,” Greg Epstein, humanist chaplain at Harvard University, says. “First of all, the numbers for young Americans are still dramatically higher, and secondly, it is beyond dispute now that the "nones" are one of the largest demographic groups in the United States, and we're going to stay that way for a long, long time.”

The Gallup study also found that 27% of Americans age 18 to 29 identified as religious nones, making the age group the largest subgroup in the study. The finding tracks with other studies on religious nones, many of which have found the growth among the religiously unaffiliated has been most notable among people who are 18 to 29 years old.

“There's no slowing here at the Secular Student Alliance. We're up to 394 campus groups from 310 a year ago,” Jesse Galef, communications director at the organization, says in response to the survey. “You can see the religious future of America just by looking at the demographics: Young Americans (18-29) are almost three times as likely to be unaffiliated with religion than senior citizens are.”

In particular, Galef points out, the Secular Student Alliance has experienced growth in ages below 18, an age group that Gallup did not survey. In the last year, says Galef, the number of Secular Student Alliance affiliates at high schools doubled to 60 campuses.

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News of strong growth among nones had long been heralded by their community.

As study after study began to report that religious nones in America were growing, many atheist, agnostic and humanist activists began to stress the need for these relative non-believers to come together and turn their numbers into political and social influence. Though some leaders split on what wielding that influence would look like, the size of religious nones became the impetus for many leaders to call for more recognition.

In response to the Gallup study, those calls continued.

“The real question now is when are our historically large numbers going to start turning into more votes and influence,” Epstein says. “The nones can become a steady and inspiring powerhouse in American life if we focus on what we do believe in.”

And even though the Gallup study found a relative leveling off of growth among the nones, David Silverman, the president of America Atheists, says he finds the survey “not at all troubling.”

“This underscores what American Atheists has been saying for years - that every person in America knows more nonreligious people than they think they know,” Silverman says. “America has to get used to the fact that atheists are everywhere, you already know us, and we are a vibrant and growing portion of society.”