Religion-based systems of discrimination in theory allow the individual to convert and become a member of the privileged class, for example the Ottoman millet system, or French colonialism in Algeria. But in practice, any attempt by the oppressed to do it at scale is resisted. Portugal traded in African slaves starting in the 15th century, saying it was legal because they were not Christian; then as the Congo converted to Christianity, the legal basis for slavery changed from religion to race.
