How do I describe a person who strongly believes in caste system? Does even a word exist to describe the same?
Since caste in India signifies a social hierarchy, different classes (though I think caste in its meaning is actually stronger than class), possibly
could be used for members of the upper caste who feel the way you describe. Usually the top of the pyramid look to maintain their status. But elitist possibly doesn't have the strong feeling that
carries. It's unfortunate that racist by definition applies only to race. If different castes were thought of as different races, it might be appropriate with all the connotation associated with it.
** In UK law, discrimination based on caste is being considered as a form of racism, the debate has been going on for years.
Other terms which might be associated with someone who believes in the caste system are
An exponent of caste is someone who promotes the ideas of the caste system
An advocate of caste is someone who promotes the practice of the caste system
There aren't many words related to the idea of a caste system in English, because such systems are uncommon in countries with English as the primary language. Probably the best choice for a person who supports a caste system would be classist. Be aware that this carries a negative connotation. Saying someone is a classist is an insult; you're saying that they support judgement, discrimination and mistreatment of others based on their social standing.
I don't know of any words or idioms which have the same meaning but express a positive opinion. The closest opposite would be something like egalitarian, meaning a person who doesn't discriminate and treats people fairly. However, this term includes more than just social standing, because it is a general term. Skin color, gender, age, etc. are all included here.
If you're trying to say something beyond just that the person supports a caste system, then please elaborate on what exactly you want to say. English has plenty of words for issuing value judgements.
You could simply use caste believer, which sounds neutral enough in my humble opinion.
And it's obvious that I'm not the first who's ever used this term. There are 7 results in Google Books (it's used as part of a longer phrase in some results) and a lot more in Google search results.