He wasn't. He had a grandparent who was Jewish, but the family had converted by the time Marx was born.

Well, yeah, Karl wasn't Jewish, but Groucho, Chico, and Harpo were, and that's probably close enough for Stephen Miller...

25% was Jew enough for the camps.

theologically, i think, marx can be considered a heretical-inverted-hegelian-christian sectarian in the embrace of the advent of modern social science. (but your mileage may vary...)

True, but Hitler was a big believer of "blood poisoning," just like someone else we know.