ORLANDO, Fla. - In his first class of the fall semester, University of Central Florida professor Charles Negy suggested to his cross-cultural psychology students that they might want to read his email to last semester's class that went viral on the Internet - twice.
In the email that created a sensation at the time and again just weeks before the fall term began, Negy chastised the devout Christian student who told the rest of the class to ignore the professor questioning their religious beliefs.
"Students in my class who openly proclaimed that Christianity is the most valid religion, as some of you did last class, portrayed precisely what religious bigotry is. Bigots - racial bigot or religious bigots - never question their prejudices and bigotry. They are convinced their beliefs are correct," Negy wrote.
The confrontation between the agnostic professor and the Christian student is, in miniature, a re-enactment of the ongoing clash in American society between strident true believers and increasingly vocal non-believers.
"I think the tension you are seeing now is the more non-believers there are, the less willing we are to accept that arrogant assumption" that Christianity is the only true religion, Negy said in an interview. "We are not going away any time soon, and the more of us there are, the more confident we feel."
About 19 percent of Americans now identify themselves as "unaffiliated" with any religion, including about 5 percent who say they are agnostic or atheist, according to a 2011 survey by the Pew Research Center. In 2008, 16 percent of adults identified themselves as unaffiliated - up from 7.3 percent when they were children, according to Pew.
Nationwide, 29 percent of Americans - and 28 percent of Floridians - say they do not believe in God, according to another Pew survey.
"There is certainly a concern about the increase of secular and non-religious people becoming more vocal," said Mat Staver, chairman of the Liberty Counsel, a non-profit law firm that advocates for Christian religious views. "What we have seen in the past few years is an aggressiveness among atheists and non-believers toward those who believe in God."
Fred Edwords, national director of the United Coalition of Reason, said the rising profile of non-believers in the United States began around 2004 with several popular books by atheists and humanists such as Christopher Hitchens. Local groups of atheists, agnostics, humanists and freethinkers began springing up throughout the nation, and a movement to unify the different varieties of non-believers started in 2009.
Edwords compares the growing size and visibility of non-believers to the gay-rights movement in its infancy. Non-believers often refer to "coming out of the closet."
"What we are finding is more people coming out. We have four or five new groups in Orlando," said Jack Maurice, founder of the Orlando Freethinkers & Humanists organization. "Coming out is a lack of fear."