“Evangelism no longer means that the job of Christians is to secure formal affiliations, shore up denominational identities, and expand Christian hegemony in Western culture. Evangelism is less and less about programming and institutions, more about relationships and authenticity.”

When Craig Ellis was growing up, he picked up the sort of adventure book meant for a boy looking to serve God. The book, “Shadow of the Almighty,” told the story of Jim Elliot, a young American evangelist killed while doing mission work in Ecuador.

The narrative of this Christian martyr did for Mr. Ellis what a superhero comic might have done for his peers: It got him pondering purpose, struggle and sacrifice. The book also provided a model for how a Christian should spread the news of salvation while working in treacherous territory, at great personal risk.

Very little in “Shadow of the Almighty,” however, prepared Mr. Ellis for where he stood on a recent Tuesday, in a room with industrial carpet and a dropped ceiling at Redeemer Presbyterian Church on the Upper West Side of Manhattan, where people lined up on Sunday morning are more likely awaiting a table for brunch than taking communion.

His weekly sessions, called the WS Café in a reference to the neighborhood, are at a new frontier of evangelism, one that seeks converts among a fervent and growing number of atheists in this country. The sessions started in September as a push by Redeemer Presbyterian’s prominent pastor, the Rev. Tim Keller, to preach the gospel to skeptics.

Such efforts proceed amid a rare moment in both Christian and American history. At the origin of Christianity, its apostles sought to convert adherents of other faiths, whether Judaism or Roman paganism. Missionaries of the last few centuries journeyed to China or Africa or the Americas to encounter the followers of other faiths, whether Buddhist or Yoruba or Aztec. In every case, the Christian evangelist seeking converts was at least dealing with listeners who embraced the concept of a divine being involved in the world.

Modern America has presented an entirely different scenario. A study last year by the Pew Research Center found that 23 percent of respondents identified themselves as “nones” — a term meaning atheistic, agnostic or religiously unaffiliated. The pop culture success of such prominent atheists as the talk-show host Bill Maher and the author Christopher Hitchens attests in a different way to the same trend.

“This falling away from faith is unprecedented in American history,” said Randall Balmer, a professor of religion at Dartmouth College. “We are, always have been, and remain, a religious people — but less so than before.” Another historian of American religion, Christopher H. Evans of Boston University, traced the current boom in nones to such smaller predecessors as the Transcendentalists of the 19th century. Both movements, he said, espoused “a belief that ultimate meaning is found within the individual, as opposed to an external God or deity.”

Against the current demographic odds, old-style evangelism must adapt. “Christian communities of faith will still be vitally important in reaching ‘nones’ and so will patterns of formation into faith,” said Bryan P. Stone, a Boston University professor who specializes in evangelism. “But evangelism no longer means that the job of Christians is to secure formal affiliations, shore up denominational identities, and expand Christian hegemony in Western culture. Evangelism is less and less about programming and institutions, more about relationships and authenticity.”

Dr. Keller of Redeemer Presbyterian has built his ministry very much on confronting the challenge. His books include “The Reason for God: Belief in an Age of Skepticism.” He periodically teaches an adult-ed class titled “Questioning Christianity” and sometimes holds question-and-answer sessions with attendees after Redeemer’s Sunday worship services.

His decision to open a branch of Redeemer on West 83rd Street in 2012 — the first new church built in the neighborhood in decades — was a brick-and-mortar way of meeting nonbelievers where they live. And he prepared his young ministers and staff members for the Upper West Side by studying together such books as the philosopher Charles Taylor’s “A Secular Age.”

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