Mr. Kurtz’s second complaint goes beyond internecine power struggles. He said that Mr. Lindsay was turning the center away from Mr. Kurtz’s humanist philosophy and toward negative, angry atheism.

According to Mr. Kurtz, skeptics must do more than just deride religion. “If religion is being weakened, what replaces it in secular society?” he asked. “Most of my colleagues are concerned with critiquing the concept of God. That is important, but equally important is, where do you turn?”

In books like “What Is Secular Humanism?” Mr. Kurtz has argued for a universal but nonreligious ethics, one he now calls “planetary humanism.” Its first principle is that “every person on the planet should be considered equal in dignity and value.” In his books, he explains how this principle can be derived from nature and from what we know of the human species.

And he contrasted his affirmative vision with recent projects under Mr. Lindsay, like International Blasphemy Day.