“Aham Brahma asmi” (I’m Universal Spirit) and “Tat tvam asi” (You are that) declare Upanishads unequivocally. Bible says “God created mankind in his own image.” Buddha says: “you are your own light”. This means that human is part of Universe; rather s/he is that Universe itself as Upanishad says: ‘That (Outer World) is Purna (Full with Divine Awareness); This (Inner World) is also Purna (Full with Divine Awareness); From Purna comes Purna , Taking Purna from Purna, Purna Indeed Remains (as Divine Consciousness is Non-Dual and Infinite).’ So we are part of Divine as well as full due to (despite!) our emanating from it. In other words all life is one. Barriers we created like religion, race, colour, Caste, etc. are artificial and against human-Divine nature. Of all the parochial forces man thought of Caste is the most heinous, most inhuman, most divisive and the most dangerous enemy to the Sacredness of Human Personality.
Also it is least amenable to explanations – rational or otherwise. For instance colour or race though not justified has, at least, some outward or identifiable differences. Caste doesn't have any: it is the parcelling out of same religious, ethnic and cultural group into alien sub-groups with visceral animosity to one another. This animosity cannot be eliminated without love born of conjugal bond. Hence the name Gandharva for it is the best of the eight marriages Hindu Shastras recognised.