If someone calls you Arab, they don’t just generally mean that you speak Arabic, but rather that a large number of your ancestors spoke Arabic, and those ancestors were genetically isolated from non-Arab speakers.
If someone calls you Jewish, they don’t generally mean that you practice Judaism, but rather that a large number of your ancestors practiced Judaism, and thus were genetically isolated from those who didn’t.
So linguistic and religious heritage are actually forms of racial heritage, but we’re not supposed to call Arabs and Jews races, instead we call them ethnic groups. Ethnic groups are subgroups within races, for examples Arabs and Jews are both types of Caucasoid, but they also differ from conventional racial categories in that the genetic isolation that formed these groups was cultural (linguistic and religious barriers) and not geographic. It was man-made constructs that prevented Arabs and Jews from exchanging enough genes to become a single population, not natural impediments like deserts, oceans and mountains, which separated Negroids, Caucasoids and Mongoloids.
If ethnic groups are a more subtle and specific form of race, then what is a more specific and subtle form of ethnic group?
If someone says you belong to a high social class, they don’t necessarily mean that you personally work in a prestigious occupation, but rather that a lot of your ancestors worked in prestigious occupations and that this isolated them, genetically and culturally from lower occupations.
Thus the concept of social class can only exist in societies with very little class mobility, because if there’s too much, it’s hard for them to solidify into genetic and cultural clusters, and without these clusters, you don’t have a class hierarchy, you merely have an occupation hierarchy.
Perhaps the only scientific way to measure social class is to ask people what each of their four biological grandparents did for a living. If most of them worked in humble occupations, then sadly you’re low class no matter how rich you are, or how prestigious your occupation, education and social circle may be.
The converse is also true. You could be a chronically homeless high school dropout and sleeping on a park bench, but if your four grandparents were all doctors, professors, or bankers, you’re high class.
Just as moving to Africa doesn’t make you black if all your ancestors are from Europe, getting a good job doesn’t make you high class if all your ancestors were low class.
So if class is a subset of ethnicity and ethnicity is a subset of race, than classism is just a more specific form of racism and commenters on this blog are correct to note the hypocrisy of liberals condemning racism while mocking white trash.