How did Christianity spread in ancient Rome

Christianity slowly evolved from Judaism. The apostles first spread the message of Jesus in what is now Israel. That was part of the Roman Empire at the time, which was huge at the time. All countries around the Mediterranean and many more were included, even England, the south of Germany and Turkey.
Later on, wandering monks told people in other European countries about the new faith. In other continents, people did not find out about it until several hundred years later, when the first ships from Europe crossed the ocean and brought all parts of the world into contact with one another.

First, many groups of people met in big cities to think about Jesus and what he had said and done. Over time, the groups got bigger and bigger. The largest Christian communities were then in Jerusalem and Rome and in Constantinople, Antioch and Alexandria. Constantinople is now called Istanbul and, like Antioch, is in what is now Turkey, Alexandria is a city in Egypt.

At some point the followers of Jesus were called "Christians". In the beginning they were persecuted. But after around 300 years, the Roman Emperor Constantine allowed the new religion. A few decades later, Christianity even became the official religion of the entire Roman Empire.
From there, many monks wandered. They set out to inspire the new faith in other countries. A few centuries later, Europeans took Christianity across the sea to America and Africa.

Unfortunately, the spread of Christianity was often not peaceful. Many people have even been forcibly forced to be baptized and become Christians. That was not at all Christian.