Paul to Constantine

Christians who were persecuted and dispersed from Jerusalem spread the Word of God everywhere they went.  The Apostle Paul and other Apostolic Fathers were largely responsible for spreading the Word of God throughout the Roman Empire.
Christians were still different from the Jews and Gentiles.  Christians did not worship idols, like most Gentiles, and they preached that Jesus was the Messiah, unlike the Jews.  The Jews thought that the Messiah would be someone who would restore the Kingdom of David and save them from the Romans.  So many wouldn’t accept Jesus as the Messiah.  Christians would not serve as soldiers because Jesus taught them to love their enemies.  During the times of peace, Christians spread the gospel and strengthened the church.

Constantine's conversion by Rubens

Christians were dispersed throughout the Roman Empire and each of the different regions had a senior bishop who served his Christian congregation and spread God’s word to non-Christians. Eventually, the Church was considered a universal church, and therefore it was called the “Catholic” or universal church.
Between 200 and 300 AD Christians recorded their ideas about Christianity.  The Christian Church had to decide which writings or doctrines they could accept as being the inspired Word of God.  By 300 A.D. the New Testament was complete.

In 313 A.D. Constantine became the Emperor of the Roman Empire. Under his rule, the Edict of Milan put a stop to the persecution of Christians and eventually made Christianity the official religion of the Empire. Up to that point, only the lower social class had been Christians, but now the Church moved into the upper classes as well. Everyone within the Empire was now known as a Christian, whether they accepted Christ as their Savior or not. Many Christians were quite satisfied with this new normal, but others were unhappy about this ‘forced’ Christianity.  As a result some isolated themselves from society, broke away from the Church, and some in society reverted back to the original paganism.

Over time, because Rome was considered the epicenter of the Church, the Pope or the bishop at Rome became the most powerful leader.