In the 1st century AD, Christianity began to spread throughout the Roman Empire. Christianity is a monotheistic religion, meaning that Christians believe in only one God. Christians believe that Jesus Christ is the Son of God and that he came to Earth to save humanity from sin. Christianity eventually became the official religion of the Roman Empire, and many of the Roman gods and goddesses were replaced by Christian saints.
However, some Romans continued to practice polytheism even after Christianity became the official religion. In the 4th century AD, the emperor Julian tried to restore polytheism as the official religion of the Empire, but he was unsuccessful. By the end of the 4th century, Christianity was the dominant religion in the Roman Empire.