Born in 1965, Mr. Selmanovic was raised in a secular Muslim home in Croatia. It had, he writes, its own religion, with two doctrines, “Thou shalt enjoy life”  which meant food, family and friendships  and “Thou shalt not be a jerk”  which meant generosity, honesty and hard work. The family feasted on spitfire-roasted lamb at the end of Ramadan, without ever having fasted. They had a Christmas tree and Easter dinner, without ever going to church. For young Samir, “life was complete,” he recalls in his book.

“Until I became a Christian, and it all fell apart.”

Mr. Selmanovic’s clandestine conversion to Seventh-day Adventism while doing his obligatory military service in the Yugoslav army led to two years of banishment from his family and a rift that could not be healed for many years. It led to theological studies in the United States and to leadership of two swiftly growing churches, first in Manhattan and then in Redlands, Calif.

It also led to spiritual crisis, a reaction against believers, including himself, tempted to feel that they had exclusive possession of God. In 2007 he returned to New York City and founded Faith House, which brings together Christians, Muslims, Jews, atheists and others to learn from one another. He also directs a small Christian community called Citylights.

Mr. Selmanovic’s thesis in “It’s All About God” is that when religions turn into “God Management Systems” pretending to own God, they turn into idolatry. The quest to find God beyond the boundaries of one’s faith, he argues, has to be moved from occasional conferences resembling interfaith prom parties (“That was really nice. Let’s interfaith again next year.”) to something central.

“Other religions can challenge (or at least help us see) the idols we create because they expand the whole territory of knowing,” Mr. Selmanovic writes. “They pose difficult questions we don’t want to ask, make assumptions we don’t want to acknowledge or examine, create meaningful arguments against us we don’t want to consider, and expose harmful practices we don’t want to stop.”