[MacNaughton]

... Marx -- who was NOT a Jew, but rather had rejected his family's history in favor of atheism. ...

The history I recall is that Marx's father was ethnically Jewish and had been practicing Judaism before he converted to Christianity. Don't recall if his mother was ethnically Jewish. But, yes, Karl Marx rejected Judeo-Christianity.

[Worldtraveler once upon a time]

--- "...ethnically Jewish...."

I am not sure if that can be actually defined. Have you a definition?