Morality depends on “the totality of facts that relate to human well-being, and our knowledge of it grows the more we learn about ourselves, in fields ranging from molecular biology to economics,” Mr. Harris told me. He has stressed the special role of his own field, cognitive science. Every discovery about the brain’s experience of pleasure and suffering has implications for how we should treat other humans. Moral philosophy is really an “undeveloped branch of science” whose laws apply in Peoria just as they do in the Punjab.

Pragmatist philosophers like Philip Kitcher offer a different approach to the question of atheist morality, one based on “the sense that ethical life grows out of our origins, the circumstances under which our ancestors lived, and it’s a work in progress,” he said. In the pragmatist tradition, science is useful, but ethical claims are not objective scientific facts. They are only “true” if they seem to “work” in real life.

“Successful experiments” — the trial and error of weighing self-interest against the needs of the community — “built the human conscience,” Mr. Kitcher wrote in his 2014 book, “Life After Faith.”

“People and societies may balance valuable things in different ways,” he told me. “A certain kind of pluralism is O.K.. But that’s a long way from moral relativism. A bedrock of ethical truth emerges and remains stable.”

The average nonbeliever may know even less about his tradition’s intellectual debates than the average Christian does — because its institutions, like Sunday Assembly, tend to be tiny, relatively new and allergic to anything that resembles dogma. But nonbelievers should pay attention. Atheism, like any ideological position, has political and moral consequences. As nonbelievers become a more self-conscious subculture, as they seek to elect their own to high office and refute the fear that a post-Christian America will slide into moral anarchy, they will need every idea their tradition offers them.

Yet modern secular humanism is also a species of 21st-century liberalism, and many of its adherents have absorbed the modern liberal tendency to shy away from ideology in favor of a message of nonjudgmental inclusion. Mr. Harris worries about any secular humanist who upholds “tolerance, above all, as the master value,” he told me. “What that person doesn’t see is that these irrational beliefs he’s refusing to criticize are of huge consequence geopolitically and personally — and are themselves sources of intolerance.”

Today, nonbelievers often seem inclined to describe atheism and secular humanism as an “identity” whose claimants should focus on winning cultural acceptance rather than intellectual debates. Here, they are taking their cues from the civil rights movement, particularly the rhetoric of gay liberation. Some organizations, for example, declared April 23 the first “Openly Secular Day,” “a celebration of secular people opening up about their secular worldview, and an opportunity for theistic allies to show their support for secular friends and family.”