Growing up, Mayor Bill de Blasio was the only child on his block who did not attend Mass on Sundays. “Everyone else was at church, and I wasn’t,” he recalled in an interview last week. “Some of the kids envied me.”

His mother, a lapsed Catholic, had little interest in organized religion, and Mr. de Blasio inherited her skepticism. To this day, he belongs to no church, and prefers to call himself “spiritual” rather than religious.

Yet as the leader of a famously secular city, Mr. de Blasio has been emerging as something unexpected: a champion of religion whose administration has advanced the cause of faith groups in the unlikeliest of public squares.

In Mr. de Blasio’s New York, public prekindergarten classes will soon be able to include a midday break for observant students to pray. Schools will be closed citywide for two Muslim holy days. He is poised to relax health regulations governing a controversial circumcision ritual that is favored by some ultra-Orthodox Jews. And the mayor says he is intent on finding a way for church groups to continue holding services in public schools on weekends, even as the United States Supreme Court could decide as early as next week to take up a case about whether the city has the right to prohibit the practice.