Isa, peace be upon him, lived without the things of the world. The early Christians who followed him were very poor and weak. The Roman Empire persecuted them mercilessly. Nevertheless, Christianity spread. The people who became Christians fastest were the slaves and the poorest of the people.
The Empire was built on slavery. Their lives were very hard. Unfortunately, they were only told about the kind of Christianity which we know is false. They were told the lie that 'Isa, peace be upon him, was crucified, and that he suffered terribly, died, was laid in a sepulchre, and then after three days rose again. This was the terrible story that the slaves were told.
Because the slaves' own lives were so difficult they were greatly moved by this awful story. They liked the thought that a man who suffered and was killed, would really rise from the dead and be someone very important on the Last Day. They liked the thought that all the weak and poor people would have revenge on the rich and powerful people. The teaching was wrong. Many of them believed in the Trinity, which is absurd.
So this false religion spread very quickly among the slaves in the Roman Empire. This rapid spread even came to make the Roman Empire unstable.
However, there were a lot of true Christians who did not believe in the Trinity. They did not believe that Jesus was a god, or a part of god. These groups and many other groups argued a lot. The arguments were so serious that the Empire could have lost its unity. Then it would have been in danger from its enemies.
The emperor at that time was called Constantine the Great. He was not a Christian, but he assembled the Christians to resolve the debate. This was called the Council of Nicaea and it happened in 325 CE. However, although the Council decided in favour of trinitarian Christianity, that didn't really simplify anything. The two sides were still at loggerheads. Constantine later even began to lean towards the Unitarians who did not believe that 'Isa, peace be upon him, was a god. Constantine is even said to have died as a Unitarian.
The trinitarians worked away at getting power over the Empire. Finally, the official religion of the Roman Empire became trinitarian Christianity. They believed that there were three gods which were really one god! Now you can see why it was not the Romans who gave the world good arithmetic! If they start with three which is at the same time one, then they are obviously hopeless at sums.
But look! At the same time as the Christians took control over the Roman Empire, the Empire took control of Christianity. There is nothing better for an empire than a good religion. It unifies the people. They believe that they are fighting for god. It is much easier to collect taxes. And taxes in the Roman Empire were quite serious. Maybe they were not as serious as today but they were high by any standards.
The Empire had split into the Western Empire based in Rome and the Eastern Empire under Diocletian, but Constantine had reunited it briefly. He rebuilt the old Greek city Byzantium and called it Constantinople and made it his capital. Later, the Goths sacked Rome itself in 410 CE. The Eastern Empire remained, and it is sometimes called the Byzantine Empire because the old name of Constantinople was Byzantium.
Although Constantinople was in Greece it was the centre of the Eastern 'Roman' Empire. The language and culture were Greek. This can lead to a confusion, because in Arabic these people are called 'ar-Rum', so sometimes they are called 'the Romans', but sometimes 'the Greeks'. Now you can see why there is some truth in both.
All of this is taking us up to the beginning of our story. The Byzantine Empire was Christian, but they persecuted all the other types of Christians, including those who really followed the revelation given to 'Isa, peace be upon him. The ones who really followed him then were very few. Today, perhaps there is no one who truly follows him except the Muslims.