Christians ended slavery. Do you think that’s a conservative simpleton’s mock-worthy bombast, embarrassing the rest of us with his black-and-white, unapologetic caricature of American history? No. It is the considered conclusion of a Nobel laureate, a former communist, a secular Jew, and arguably the foremost scholar on American slavery.

Robert Fogel (1922-2013), the son of Russian Jewish immigrants, was president of Cornell University’s American Youth for Democracy, investing eight years promoting communism. Meanwhile, he married Enid Morgan, an African-American woman, consequently suffering the ugliness of American racism personally. Eventually, he rejected communism. Apparently, the data didn’t support it.

Fogel was driven by data, perhaps the purest pursuer of empirical truth I’ve ever met in academia. He pioneered an approach to history, called “cliometrics,” that relied on quantifiable evidence; that is, countable documentation. Like any historian, he read diaries and pamphlets for the color they throw on the times, but for his conclusions, he sought hard facts. For example, whereas other historians might look to the likes of Fanny Kemble, a British actress who married a Southern plantation owner, to piece together the lives of the slaves, Fogel believed such sources were too filtered through bias. Campaigners, like Harriet Beecher Stowe...