By the beginning of the twentieth century, it had become harder for many Americans to believe in God. Religious groups struggled to adapt to rapidly changing cultural and scientific developments that seemed to challenge the plausibility of traditional beliefs. In God-Optional Religion in Twentieth-Century America, Isaac Barnes May focuses on the stories of three groups-liberal Quakers, Unitarians, and the forerunners of what would become Reconstructionist Judaism-that attempted to preserve their faith in the modern world by redefining what it meant to be religious. Between the 1920s and the 1960s, these communities underwent the most massive theological change imaginable, allowing their members the choice of what kind of God they wanted to believe in, or the option to not believe in God at all.

These groups pioneered the idea that being religious and believing in God might be separate concepts, a notion that spread widely, moving from church pulpits to novels and magazine covers. Eventually, the Supreme Court enshrined the idea that "God" could mean many different things in American law. God-Optional Religion in Twentieth-Century America provides an intellectual history that helps make sense of why most contemporary Americans' answer to whether they believe in God is often far more complicated than a simple "yes" or "no."