However, secular activists in America are still a long way from that result, and they need to target their outreach better if they hope to get there, Campbell said. He estimates that only around one-third of the religiously unaffiliated are "actively secular" and able to be drawn into a recognizable voting bloc.

"We speak of the religious 'nones' or the unaffiliated population as though they're one large group," he said. "But there is a very clear divide within that population."

Actively secular Americans search for truth in philosophy or science, finding meaning in mostly nonreligious sources. Passively secular adults, on the other hand, aren't very concerned with life's big questions and may still cite the Bible as a source of inspiration, he added.

Two-thirds of unaffiliated Americans still believe in God and around 1 in 5 say this belief is a necessary part of being a moral person, according to Public Religion Research Institute.