When I was a new believer seeking reasons for my faith, I inhaled the writings of Christian apologists such as C.S. Lewis and Josh McDowell. The way my unsophisticated mind summarized much of their material was something along the lines of, "Christianity is different from all other world religions because it is the only one in which we don't have to earn our way to heaven, because salvation is by faith, not by works."

Boston University's Stephen Prothero no doubt would have challenged my simple formula on a number of levels, such as the obvious one that Christianity includes a prominent place for works, and other religions also have faith—that is, trust—in something at least. One point we would have wholeheartedly agreed on, however, is that Christianity indeed differs from all other faiths.

But Prothero would have gone further: all the world religions, in fact, differ one from another. Prothero dismisses as sentimental nonsense the theories of many other religion scholars, such as Huston Smith, that the various world religions, though different from one another in outward ephemera, share at their core the same spirit, that they are "different paths up the same mountain."

These hopeful but ungrounded views, Prothero says, result from the influence of religious pluralism, a reluctance to argue, and a desire to homogenize religion in order to dampen violent fanaticism such as the kind that brought down the Twin Towers: "One purpose of the 'all religions are one' mantra is to stop this fighting and this killing."

In a previous book, Religious Literacy, Prothero showed how clueless many Americans are about religion—a finding confirmed by a survey last fall from the Pew Research Center: most Americans flunked the quiz on Bible knowledge, world religions, and the role of faith in public life. In his latest project, God Is Not One: The Eight Rival Religions That Rule the World—and Why Their Differences Matter, Prothero seeks to overcome our ignorance with an armchair guide to the major world religions, a job he undertakes with a certain sense of urgency: "naïve theological groupthink," he argues, has made the planet we share more dangerous. I wonder, however, whether an informed awareness of our differences really makes us less likely to fight. Maybe it does, maybe it doesn't. It could even make conflict more likely. I, for one, seriously doubt that the Islamists seeking to overthrow the West will be pacified by reading this thoughtful and engaging volume.

While pointing to salient religious differences, Prothero freely acknowledges that all religions agree there is something wrong with the world, that it is somehow "out of balance." But they disagree both on the diagnosis and on the cure. For example, Prothero would have disputed my belief that followers of other faiths are seeking salvation of the kind held out by Christianity—that is, a spiritual deliverance from sin, ultimately leading to the bliss of heaven and avoidance of the other place. Prothero insists that most religious adherents—at least those outside Christianity and Islam—aren't even looking for heaven. "The world's religions are clearly related," he writes, "but they are more like second cousins than identical twins. They do not teach the same doctrines. They do not perform the same rituals. And they do not share the same goals." Amen.

That being so, he suggests, each religion must be understood according to its own ground rules, much as the goal of baseball is to score runs, the aim of football is to score points, and the focus of hockey is to score goals. We all want to score, but we all want to score different things; it would be absurd to criticize a baseball team for failing to score goals.

In perhaps the most helpful contribution of God Is Not One, Prothero provides a template to discern the differences. He says each global faith points to a key problem confronting the human race and the world (in Christianity, sin), proffers a solution (in Christianity, salvation), presents a technique toward that end (in Christianity, a combination of faith and good works), and provides exemplars who have lived out the faith (in Christianity, saints, or ordinary Christians). This template, while necessarily oversimplified, is a great tool that allows us to quickly grasp the different essences of the world religions. Such an approach could aid missionaries in exegeting other faiths, with a view to contextualizing the Christian message in terms that connect. But Prothero, who describes himself as "religiously confused," never takes the next step up the mountain to ask the ontological question about each faith: Is it true? Instead, he asks the practical question: Does it work?