The primary religion that spread throughout the Roman Empire during its peak was Christianity. Initially a small Jewish sect, Christianity gained resonance through its emphasis on salvation, community, and moral teachings. The Apostle Paul was instrumental in spreading the faith beyond Jewish communities into the Gentile (non-Jewish) world, establishing Christian congregations across the empire. As the religion grew, it faced both persecution and acceptance, culminating in the Edict of Milan in 313 CE, which granted religious tolerance for Christians. Eventually, Christianity became the official religion of the empire under Emperor Theodosius I in 380 CE, profoundly influencing Western civilization.