The point where caste defines purity. Classism is a separate axis from the notion of religious purity that is a fundamental part of caste (which is effectively an amalgamation of the two). As such, the closest "reference point" for a U.S. citizen is the attitudes held by antebellum slaveowners regarding race. You can't comprehend the sort of...emotions that inspire the strict anti-miscegenation of racism or caste-ism, from a lens of "knighthood in the olden times".

Nevermind that one can be "knighted" - the idea that one could be "brahminated" is no different from thinking one could be "whited" in the antebellum south. (And no, the "honorary whiteness" given to certain asian ethnicities is not akin to "whiting" - it's merely defining a subdivision of a caste to be slightly higher than the other lower castes)

At no point. Indians have a distorted view of caste because of 400 years of British invasion after 500 years of Islamic invasion.

Since economic mobility was greately reduced because of economic stagnation caused by centuries of invasion, they re-applied the originally profession-based caste system using racial discrimination brought to India by the Mughals and the British.

If we're being strictly anthropological Western 'racism' is a type of caste system but all caste systems have the trait of assigning better or worse outcomes to disparate breeding pools, however defined, and re-enforcing breeding rights on some level. Almost all human societies are caste systems of some sort since the introduction of farming at least, and from a more modern perspective is a composite of multiple, simultaneous caste systems (usually referred derisively as 'isms').