After the World War II and all of the terrors witnessed through it on ethnic and racial basis, the international community was determined that something like that should never happen again, and that all humans are equal and deserve the same rights.
That benefited the African American population in the United States a lot, as they now had the basis and support with which they were able to push through to gain all of the rights they deserve, thus be equal with all of the rest. That led to the formation of multiple organizations for the rights of the African Americans, widespread propaganda, protests, and eventually it gave a positive result, with the African Americans gaining all of the rights they deserved, but also putting the racism aside in the American society.
The policy of the Roman Empire regarding religion was that people were permitted to keep their religion as long as they worshiped the emperor and the roman gods.
