The antisemites like to point that out when they discuss Marxism, since Marx, although he was atheist and his father Heinrich (born Herschel Levy) was a convert to Lutheranism, had a paternal grandfather (Marx Levy Mordechai) who was rabbi of Trier. So all “Bolshevism” they falsely characterize as Jewish, assigning guilt by association via their racist outlook.
The fact that anti-Semites point this out doesn’t change the fact that the state of Israel, among other authorities, recognizes the Jewish ethnicity independent of religious belief or practice.
His seminal essay, On the Jewish Question outlins how antisemitism is going to be backed into his philosophies:
What is the worldly religion of the Jew? Huckstering. What is his worldly God? Money.…. Money is the jealous god of Israel, in face of which no other god may exist
And henceforth, leftism/communism wasn't good for Jews.
Chief Rabbi of Moscow Jacob Maze said, "The Trotsky's make the revolution, the Bronsteins pay the bills." (The was a reference to Leon Trotsky, whose family name was Bronstein)