Historically it's a matter of power. The same way colonization generally works. Those that submit to the forced religion are given privilege and power, especially access to food and medicine. Then it's perpetuated by giving them power specifically over others, and in order to maintain that power they have to demonstrate adherence and conversion of others. It leads to class-based stratification and a superiority complex, which leads to buying into the religion, if only to maintain that power and privilege and to justify their actions against their own people.
There's also the good old fashioned death penalty and forced indoctrination of the young.
mnemonikos82 t1_je8u7zo wrote
Reply to Eli5: How does forced religion conversion work? by atuljinni
Historically it's a matter of power. The same way colonization generally works. Those that submit to the forced religion are given privilege and power, especially access to food and medicine. Then it's perpetuated by giving them power specifically over others, and in order to maintain that power they have to demonstrate adherence and conversion of others. It leads to class-based stratification and a superiority complex, which leads to buying into the religion, if only to maintain that power and privilege and to justify their actions against their own people.
There's also the good old fashioned death penalty and forced indoctrination of the young.