The reason why Christianity took hold of the Ancient world because it offered a equal, peaceful and happy life. This is the first reason of why people wanted to join Christianity. First Christians have peace using Justice while the Romans have peace but use war.
How Christianity was preserved by the Roman Empire?
In 313 AD, the Emperor Constantine issued the Edict of Milan, which accepted Christianity: 10 years later, it had become the official religion of the Roman Empire.
C permanent latin christian kingdoms in the middle east hope this helps