Christianity began in the Roman Empire. When Christianity was new, Christians were hunted as criminals. They refused to worship Roman gods and that was against the law.
Even though Christianity was against the law, its numbers or followers grew rapidly. Christians were always trying to convert people. While some Christians were rich, most of the converts in Rome were from the poorer section. There were two reasons for this.
First, Christians promised that if you lived your life right you went to heaven when you died. In the other Roman religion, only the gods could live in heaven. Everyone else was consigned to the underworld.
Second, every Christian considered themselves equal to every other Christian. There was no nobility or higher caste. (See plebians and patricians.)
In 313 CE, Emperor Constantine converted to Christianity and made Christianity legal. Under his leadership, Christians could no longer be arrested just for being Christian. After a fairly short period of time, Christianity became the dominate religion of Rome. After that, it became the law that you must be Christian to be a citizen of Rome.