How was the ideology of Karl Marx shaped by his Jewish background? Over the past several years, Yale University Press has published a series of short biographies of eminent Jews, from Moses to Barbra Streisand. This project now has provided the much-honored Israeli political scientist Shlomo Avineri with an opportunity to tackle this question, which inevitably leads to a still thornier follow-up: Was Karl Marx Jewish at all?

The answers are neither easy nor obvious. Marx was born to a Jewish mother, but his father had converted pro forma to Christianity, in order to continue his legal career at a time when Prussia barred Jews from that profession. Young Karl was later baptized, but we don’t know whether he was understanding or ashamed or contemptuous of his father’s apostasy, because he left no record of his reaction. Marx never publicly discussed his own Jewish identity and, though both his grandfathers and an uncle were rabbis, Mr. Avineri reports that he apparently knew little about Judaism as it was actually lived. Late in life, Marx enjoyed conversations with the pioneering Jewish historian Heinrich Graetz, but what they said is a mystery. The paper trail of his thoughts on Judaism in general is thin, contradictory and sometimes troubling.

His 1844 essay “On the Jewish Question” is notorious. Marx consistently supported full legal equality for Jews, but here he broadcast a classic anti-Semitic smear: that Jewry constituted a vast global capitalist conspiracy. The true religion of the Jew was sacher(huckstering). “Money is the jealous God of Israel, in the face of which no other God may exist,” Marx wrote. “In North America”—where in reality there were at that time very few Jews—“the practical domination of Judaism over the Christian world has achieved its unambiguous and normal expression that the preaching of the gospel itself and the Christian ministry have become articles of commerce.” Or as someone else more recently put it: “It’s all about the Benjamins baby.”