His religion is marxism. Unlike most politicians, wikipedia has no (nominal) religion listed for him.
Bill de Blasio opened up about his nontraditional religious beliefs during the final day of his mayoral campaign.
"It's obviously too big and too complicated a topic to try and talk about in a press conference," de Blasio said, outside the Bay Eden Senior Center in the Bronx. "You know, I am not affiliated with any particular church."
"I do consider myself a spiritual person," he continued. "As I've said many times I was very influenced by Christian liberation theology and the work I did after college and after graduate school."
He said his "family tradition is Catholic" but that his mother did not raise him as a Catholic.