The Roman Empire was way too big. Diocletian figured they needed another emperor to help. So, what diocletian did is he split the Roman Empire into two pieces, the West and the East. Diocletian ruled the East and another ruler ruled the West. They were worried that other people would not obey them, so they told people they were gods. Because Christians worshipped one God, Diocletian persecuted them. He persecuted them by hurting them, putting them in prison, and making it illegal to be Christian.
But then....after Diocletian retired people stopped persecuting Christians. 7 people shouted that they wanted to be ruler. But Constantine won. He saw a vision of a flaming, firey cross. Then he told his soldiers that all of them must have the letters of Christ's name and a cross on their shields. Then they went into battle and they won. Constantine became the ruler and a Christian. Constantine moved the capitol to Byzatium and then he renamed it Constantinople.