I'll give you 1,000 dollars if you stay a soldier.
After Jesus rose from the dead, he send his friends and discipled to spread the gospel. Some many emperors hated Christians, Nero was the first person to kill Many emperors after Nero also persecuted Christians, they would make them walk over nails, hang them by their hair, and use them as torches to light up their parties.
Constantine was a Roman emperor who was a Christian (most think), but didn't understand Christianity. At first he gave the people freedom of religion, but eventually he became so fond of Christianity that anyone who wasn't a Christian was to be persecuted. This made it to where everyone was saying they were Christians , but no one really meant it.
The Roman Empire fell because the Roman emperors kept getting assassinated, so there was no stability in leadership. The emperors were careless with their money, so they would get money that was less valuable, and it eventually got to the point where Roman money wasn't accepted for taxes. Also, the Germanic tribes invaded and attacked Rome.