Karl Marx was born and raised Lutheran. In all of his voluminous writings, there is no indication he had any self-awareness of being Jewish.
Marx’ ‘On the Jewish Question’ could have been written by any common Jew-hater of the times. Nothing personal in it.
"Karl Marx, the son of Hirschel and Henrietta Marx, was born in Trier, Germany, in 1818. Hirschel Marx was a lawyer and to escape anti-Semitism decided to abandon his Jewish faith when Karl was a child. Although the majority of people living in Trier were Catholics, Marx decided to become a Protestant. He also changed his name from Hirschel to Heinrich."
https://www.jewishvirtuallibrary.org/karl-marx