The non-religious have become the fastest growing "religious group" in the United States.

Last October, a poll conducted by the Pew Research Center found that one-fifth of Americans reported having no ties to any religion. Despite a surge in the religiously unaffiliated, the only four percent of respondents identify as atheist.

Todd Stiefel, founder of the Stiefel Freethought Foundation, has spent more than $3.5 million to promote equality for these "free thinkers" and make atheism more acceptable to the public. Non-theists, "whether they be humanists, atheists, agnostics, or otherwise," are among the most discriminated against groups, Stiefel says.

A 2012 Gallup survey asked, "If your party nominated a generally well-qualified person for president who happened to be ____, would you vote for that person?" The survey found numbers in the 90s for Jewish, Catholic, and African Americans. But athiests were found at the bottom of the list at 54 percent, below Muslim and gay or lesbian.

"In other words, almost half of the population would vote against us just for being atheist. And frankly, I want to change that," Stiefel says.

Over time, there has been a history of negative branding of atheists, Stiefel says, and though atheists have civil rights, it does not mean they have civil equality.

"We don't get burned at the stake any longer, but we were burned at the stake for thousands of years," he says.

There are seven countries right now where atheism is punishable by death, Stiefel says. And in the U.S., there are seven states where atheists are prohibited from holding public office. Those provisions might be unenforceable, but it is a reminder that this group is seen as second-class citizens in the U.S., he says.

Stifel has spent his own money to promote atheism but also to strike down discrimination against atheists. The wrong approach, he says, is to attack religions. There are, however, certain aspects of any idea system, including atheism, that should be criticized.

"One thing that free thinkers hold very high up on our value list is the concept of questioning and having doubts and not holding any ideas sacred," he says.

Stiefel agrees there are aspects of religion that are great and positive, but says that problems arise when religious texts are taken too literally.

"Our moral code needs to be updated to the modern world and updated based on what we've learned from history," he says.