Mr. DeWitt acts so much like a clergyman because he was one.

He was raised Pentecostal in DeRidder, La., a small town near the Texas border. In 2011, after 25 years as a preacher, he realized he had lost all connection to the religious point of view that had defined most of his life. He left the church and found himself ostracized in his hometown and from his family. Since then, Mr. DeWitt, 43, has become a prominent advocate of atheism, giving lectures around the region and providing an emotional counterpoint to more academic atheist exponents like Richard Dawkins and Christopher Hitchens.

With Sunday’s service — marking the start of Community Mission Chapel in Lake Charles, which Mr. DeWitt called a full-fledged atheist “church” — he wanted to bring some of the things that he had learned from his years as a religious leader to atheists in southern Louisiana.

The percentage of religiously unaffiliated Americans appears to be on the rise. A 2012 Pew Research Center study found that while only about 6 percent identified as atheist or agnostic, they were among nearly 20 percent classified as religiously unaffiliated. That was up from 15 percent in 2007, a greater increase than for any traditional faith.

Mr. DeWitt counts himself among the hard-line atheists, but he believes that something may be lost when someone leaves the church — not just the parts about God, but also a sense of community and a connection to emotion.

“There are many people that even though they come to this realization, they miss the way the church works in a way that very few other communities can duplicate,” he said in a phone interview. “The secular can learn that just because we value critical thinking and the scientific method, that doesn’t mean we suddenly become disembodied and we can no longer benefit from our emotional lives.”