Some are adopting rituals themselves, from de-baptisms to wedding ceremonies, as a way to cement ties among members. Others are organizing science-related outings or enrolling in community-service programs. Nationwide, atheists' groups are now treading, sometimes gingerly, into unfamiliar territory.

"This is the transition moment right now," says Dale McGowan, author of "Parenting Beyond Belief: On Raising Ethical, Caring Kids Without Religion." "Some groups are really diving in [to foster a robust sense of community], and some of them are holding their noses and standing on the diving board. They're not quite sure what to do." ...