Updated: Jun 17, 2019
Here's my take on the spiritual concept underlying the exoteric Hindu Caste System. The social institution is a pseudo-sophisticated, bumbling and relatively evil symbolism of this unifying concept. It is still relevant, albeit to varying degrees, in most parts of the sub-continent.
I'm not very sure of the origins of the Hindu Caste System and I don't really care: it's just another of man's half-arsed attempts at establishing "heaven on earth" despite possessing infinitesimal knowledge of The Infinite. What I do know is that ancient Indian emperors, between 300-175 B.C., like Chandragupta Maurya and his grandson Asoka rejected the caste system and preferred the Golden Rule as a guideline for social behaviour.
For the record, the Mauryan Empire is the only known geopolitical structure of ancient times that somewhat resembles the modern republic. Chandragupta was from a low caste and is credited with having driven the Greeks out of India after the death of Alexander The Great. The caste system was reintroduced as social principle after the fall of the Mauryan Empire.
It is only fairly recently, over the past 250 years or so, that the system began to lose its hold on South Asian society once again. This was driven by different things: external influences like the Persian and British invasions; the social, spiritual and scientific visionaries--quite a few Brahmans in this group--of this period and the general evolutionary trend towards a wider and more encompassing consciousness.
Like most symbolic traditions, the physical symbol of the concept is often a stumbling block to the concept itself. This is what happens when the ego tries to make Spirit conform to its own highly-limited image.
In truth, the caste system is about different states of consciousness. It is not about humans and their egoistic preferences, but about divine states of existence. These states are not the property of any individual or collective and are flexible. Any individual or collective may, for a time, fall into such states and experience their consequences for as long as they remain attached to the forms of the state.
It is not a tool for any rigid social hierarchy. To use it as such is asking for "trouble" in accordance with the Law of Cause-and-Effect. It is rightly written in the Bible:
"And mine eye shall not spare thee, neither will I have pity: but I will recompense thy ways upon thee, and thine abominations shall be in the midst of thee: and ye shall know that I am the Lord." (Ezekiel 7:4-6 KJV)
Those are not the thoughts of some bearded dude in the sky but a theatrical description of an impersonal and eternal principle.
Man's machinations, although divine on one level, are in complete subjugation to the Higher Self, i.e. God/Consciousness and Divine Law.
In principle, the Brahman should be one who knows his unity with all things--including the so-called Untouchable. The Brahman, more than any other individual, must be fully aware that he is no better or worse than anything else. The Brahman realises that tradition is irrelevant and that state of consciousness is all-important. The Brahman is certain that all things external to him---whether vile or pleasant--are an aspect of his very own Self.
A true Brahman would advocate a flexible caste system where an individual would be rewarded according to his state of consciousness, as is always the case anyway, not the man-made travesty of social structure. The symbolic and individual Brahman, however educated and accomplished he may be, is never permanently assured of this position unless he permanently dwells in such a state of mind, i.e. unity with all things. If the Brahman descends in consciousness and is attached to an illusion, like the social caste system, he will experience the corresponding consequences that will, sooner or later, be persistent physical conditions.
That said, let the Untouchable also realise his Divine Nature and oneness with the Brahman. Let the Untouchable not mire himself in any form of inferiority, superiority or spite towards anyone external to him. Instead, let him turn within, seek self-knowledge and in so doing, realise his true nature as Brahman. In such a state of consciousness, the Untouchable is a Brahman. In short order, the physical Untouchable abiding in the state of Brahman shall emerge into corresponding physical conditions.
By corresponding physical conditions, I don't necessarily mean the dead social symbols of the caste but the more general aspects, i.e. a Brahman may find himself in a pitiable social position of some sort and the Untouchable may rise to the heights of society.
Let none think himself to be above God. Let all realise unity with God who is omnipresent, i.e. all things simultaneously.
Let all, whether Brahman or Untouchable or anything else, be fully aware that anything in their experience, however pleasant or painful, is an aspect of themselves in a different form.
Any egoistic assumption of superiority or inferiority will invite divine discipline.
No social order, regardless of apparent power, can withhold Divine Law. Of this, I am sure.
Consciousness is God. God is the only reality. God is not mocked. All is one and one is All.
I suppose that might be one of the advantages that Christianity has: as a spiritual system it cares little for social norms and appearances. Hinduism, and possibly Judaism (: although that might be purely allegorical :), seems to have mixed the "dead" and transient with the living and eternal spiritual principle. To be fair, any form spiritual necrophilia is naturally aroused by the noxious stimulants emanating from tradition's perpetually decaying corpse.
A superficial understanding of any system leads to its socially egoistic and rigid implementation that invites the automatic consequences--usually unpleasant--of the Law of Cause-and-Effect. Nothing in duality overrules the Law save through the Promise or knowledge of Grace (: your True Self :).
Let us all be constantly reminded, whether pleasantly or harshly, that there is but One God and all are aspects of the same. The Brahman is a Brahman and an Untouchable. The Untouchable is an Untouchable and a Brahman.