In searching for the keys to religious people’s “madness,” Dr. Offit cites celebrity researchers like Robert Jay Lifton on cults and Stanley Milgram on obedience. In one devout Christian, Larry Parker, he diagnoses narcissistic personality disorder. In throwing away his son’s insulin to show his trust in God, Mr. Parker certainly employed bad judgment; he and his wife deserved more than the probation they were given by a sympathetic judge. But it’s not helpful to use psychiatrists’ language — “a pervasive pattern of grandiosity,” from the Diagnostic and Statistical Manual of Mental Disorders — to pathologize those who think they know God’s will.

On Judaism, Dr. Offit is no better. He writes that the Jewish commandment to circumcise sons is “second only” to “be fruitful and multiply.” This is true if “second” refers to the order in which the commandments appear in Genesis. But it’s not true if he is, as he seems to be, discussing their relative importance, because the commandments aren’t ranked.

Recounting the practice, upheld by some ultra-Orthodox Jews, of sucking the blood off an infant boy’s penis after circumcision, he writes that in the 19th century “most rabbinical authorities” followed one Rabbi Moses Schreiber in abolishing the practice. I checked this citation, which was to a 2004 article in Pediatrics. This article indeed backs up Dr. Offit’s version; but that article in turn relied on a 1972 article in the Jewish journal Tradition, which does not say that “most” rabbis followed Rabbi Schreiber. In fact, the Tradition article seems to say that, if anything, the rabbi was in the minority.

Near the beginning of his book, Dr. Offit writes that as an irreligious man he had expected to find the Bible culpable for the harm committed in its name. “But I didn’t,” he says. “Somewhere during the process of reading large sections of the Old and New Testament, I changed my mind, finding myself largely embracing religious teachings.”

He seems to have adapted a kind of liberal Christianity — “What I have learned is that to be truly religious is to be humane,” he writes, vaguely — and concluded that, where religion undermines science, fundamentalism is to blame. It’s a crude, oversimplified argument.

And Dr. Offit’s paradigm ultimately doesn’t explain very much. The anti-vaxxer community, to take the obvious example, includes libertarians, conspiracy theorists and New Agers who are deeply skeptical of biblical religion. Irrationalism, including the fear of science, courses through America’s veins, and it’s only sometimes religiously inspired.

Few doctors have done more than Dr. Offit, in clinical work or in writing, to save lives by encouraging vaccination. Had he striven for a more nuanced understanding of religious people, and had a tough editor prodded him to a make a more sophisticated case, “Bad Faith” could have furthered his life’s work. Instead, this book feels like a rush job.

Dr. Offit has made a career as a children’s crusader, using the scientist’s armament of reason and research to defeat slack thinking and superstition. How sad that, in this fight, he left his best weapons at home.