The worst use of religious authority to subjugate people is the caste system of India and the Hindu tenet of reincarnation. The idea of reincarnation has entered US beliefs along with other elements of Eastern religion, such as Zen and Tantra. It also appears in Plato's myth of Er. But the idea of reincarnation has had its biggest effect in India. There, the point of reincarnation is to support Hinduism's oppressive caste system.
The Brahmins may not have been the worst priests ever. The Aztec priests, with their bloodthirsty sacrifices, may take that honor. But the Aztec priests ruled a small number of people for a short period of time compared to the Brahmins. In terms of scope, if not intensity, the Brahmins have been the worst.
Aryans imposed the caste system on the native Indians when they invaded and conquered the subcontinent. The caste system divided the social superiors into the three classes typical of Aryan culture: priests, warriors, and commoners. You see the same divisions in medieval Europe. The natives, however, make up a fourth caste, the darker-skinned shudras, born into lives of servitude. These four groups are known as varnas, each composed of numerous castes. A fifth group, outcasts, has it even worse.
To lock down the caste system, the priests foisted the idea of reincarnation on the populace. Reincarnation bolsters caste oppression in two ways. It justifies injustice, and it deflects hopes for progress from this life to an imaginary next life.
For the people on top of the caste system, the brahmins, reincarnation justifies why they should enjoy privileges of high-class birth. They must have earned those privileges through virtuous behavior in past lives. A privileged birth proves that one deserves privilege. For the people on the bottom, the shudras and untouchables, reincarnation justifies why they should suffer for their low birth. They must have earned their suffering through sinful acts in past lives. Reincarnation provides an oppressive caste structure with the trappings of a meritocracy.
The idea of reincarnation also tricks low-born Hindus into supporting rather than opposing their own oppression. Hinduism teaches low-caste people that the way to improve their lot in the next life is by leading a virtuous life this time around. Conveniently for those who invented these beliefs, virtue for a low-born Hindu means serving the high-born.
The connection between reincarnation and the caste system shows up in the negative in the attitudes of the Charvakas. The Charvakas were materialist, atheist contemporaries of ancient Hindus and Buddhists. The Charvaka school of thought rejected what the adherents couldn't perceive, which made them immune to the inventions of the priests. They believed in the four material elements (air, earth, fire, and water), but not the fifth (empty space or ether). They discounted both reincarnation and the caste system. It may be that the Charvaka philosophy represents the indigenous belief system, present before the Aryans invaded. Another term for Charvakas is Lokayata, which is translated either as "worldly" or "prevalent among people." The pre-Aryan Harappa culture had no temples, which probably means no priests to tell people to believe something other than what they could perceive for themselves.
Hindu priests aren't the only ones to invent "past life" justifications for real life injustice. For example, Mormons teach that blacks were cursed for their poor showing in the pre-existence. Oppressing blacks was justified until Elohim forgave them in 1978. Similarly, northern Europeans are superios to southern Europeans due to their exemplary lives in the pre-existence.
Hindu priests are also by far not the only ones to invent an afterlife to exercise control over others, especially the common classes. Even today, for example, Catholic bishops threaten to hurt political opponents' afterlives by denying them Communion.
Reincarnation, however, is noteworthy first for bookending the faithful between past and future lives and second for the vast number of people that have suffered under the tenet over the last few thousand years.
Reincarnation makes unfairness seem fair and makes service to the high castes seem like a ticket to being reborn in a higher caste oneself. Supporting India's caste system, the brahmin con job has oppressed more people in history more viciously than any other religious scheme. In its audacity and success, for example, the brahmin deception puts the Catholic sacraments to shame.