When Barack Obama delivered his first inaugural address six years ago last week, on January 20, 2009, it was the first time a newly elected president used the occasion to give voice to the diversity of religious life among its people. “We are a nation of Christians and Muslims,” Obama said, “Jews and Hindus, and nonbelievers. We are shaped by every language and culture, drawn from every end of this Earth.”

Such a high­-profile expression of the varieties of American religious experience was unprecedented, even if the reality it described predates the Republic itself. A spectrum of beliefs informed the nation’s history well before its first president, in his 1789 inaugural address, spoke of “that Almighty Being who rules over the universe.” Yet Obama’s choice of words served as a reminder that only recently did the range of opinions about the nature of that Being, including its existence, begin to receive their due in the ongoing national conversation about the appropriate place of religion in American life.

Perhaps most noteworthy about the president’s acknowledgment that the United States is a country of many faiths was that it seemed noteworthy at all. His simple declaration of a catalogue of beliefs surprised many because there persists, among believers and nonbelievers alike, an assumption that the United States is, for better or worse, a Christian nation.

Nothing has done more to keep this notion alive than the stubborn persistence of words spoken more than a century before this land was a nation at all: John Winthrop’s designation of the community he would establish in America as a “city upon a hill.” Some historians have doubted these words were ever spoken, but tradition maintains that in 1630, while the Puritan settlers of Massachusetts Bay were still aboard the Arabella, the governor of the colony delivered a sermon comparing their humble settlement to a lofty city described in a parable of Jesus.

For at least the past fifty years, that single unifying metaphor has dominated presidential rhetoric about the nation’s self-understanding, causing an image borrowed from the Gospels to become a tenet of faith in America’s civil religion. While not a direct refutation, Obama’s statement of religious diversity presented a challenge to reconsider the meaning, and even the relevance, of this image in the twenty­-first century. ...