Roman Gods: For many hundreds of years, the Romans worshipped thousands of gods. Trees, rocks, streams, bridges, everything in ancient Rome had a guardian spirit or god watching over it. Each god had at least one job to do. Some gods had very important jobs like watching over the crops, or bringing the sun out each day. But all gods were important. The Romans prayed to their gods at least once every day. As Rome expanded and grew, the Romans came in contact with people who worshiped other gods. If those people had fought extremely well, the Romans figured that their gods must be pretty good, so the Romans adopted those gods and made them Roman gods as well. They gave those gods Roman names and changed some of the personalities of these gods to better fit the Roman way of life. However gods were added to the Roman list of gods, it was the law that all people in the Roman world must worship Roman gods.
Christianity began in the Roman Empire. Christians refused to worship Roman gods, and that was against the law. Christians knew that was against the law, so they met in secret. If discovered, they were arrested and treated as criminals. Many were killed. In spite of the risks, many people converted to Christianity. This new religion's followers grew rapidly. There were two major 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. This sounded awfully good to the people who lived in the poor sections of Rome, in many cases good enough to take the risk. (See plebians and patricians.)
Things changed for Christians in 313 CE, when 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.