They were being forced to pay the church with goods and they were being made to do manual labor, while often times being physically abused.
Originally, Ancient Rome practiced politheism, however poor people and slaves gradually started to believe in Christianity as a way of salvation to they miserable lives. The expansion of Christianity could not be stopped by the Emperors, which resulted in Emperor Constantine made it legal in 313 AD and eventually, the workship of other gods was made illegal in 391 AD.
The Byzantine empire was located northeast of the arabian peninsula