The Upanishads were written around 800-400 BCE as a response to the abuse from the Brahmins. Therefore, they were created to establish a religion that could be practiced by everyone individually and to articulate the underlying philosophical beliefs. The paramount idea underlying Hinduism is Brahman - the World Soul - the final and ultimate reality. It is this divine entity that goes further than all other physical or even spiritual ideas and ties it all together. It is the real truth. What humans experience physically is an illusion, but the human soul, called an atman, is part of Brahman and very much real. The final quest of an atman is to join with Brahman and to remove itself from the illusion. This ultimate and enlightened end is called moksha.
Joining a soul with Brahman in moksha is no simple task. In fact, it takes many human lifetimes for the possibility a soul achieving this union, which spurred the significant Hindu belief of reincarnation. This allowed a soul to travel from body to body after its physical death and continue its quest, but that depended on the actions of the previous lifetime. This became a concept called karma. If one’s actions in life were deemed as good and pure, the soul would be reborn into a higher caste, thus the caste system becoming a socially and spiritually important construction. With that tenet, being born into a higher-ranking caste is a direct result of good karma in the previous life putting the soul one step closer to achieving moksha. Moksha is similar to the Buddhist idea of enlightenment in that is a spiritual liberation, a breakthrough of sorts, into the realm of the ultimate truth – but how one reaches it, through knowledge or meditation for example, is individualistic. This quest to unify one’s atman with Brahman is a driving force for purity and goodness extends throughout Hinduism.