Marx's parents converted before he was born and he was raised as a lapsed Protestant, so he was never Jewish, except ethnically. His "Money is the jealous god of Israel," was the root cause of the left's antisemitism.
Dear nick,
As other European societies have determined in the millenia before I was born, ‘if the mom is jewish, so was the child’. That did not delineate between ‘practicing/nonpracticing’, just bloodlines. So, Marx was a Jew, and he ‘voted’ left, by his writing ‘Das kapital’, aka, the Communist manifesto.