During the Pax Romana, travel was possible, and missionaries (who spread Christianity to non-believers) felt comfortable and safe when using the roads, and could spread their message without fear of attack.
There were open routes throughout the Empire, and people were more interested in learning the teachings of Christ during a time of peace than they would be during a war or famine- it was big news.
Pretty much everyone spoke the same language at the time, as well, so the missionaries could preach easily to the people.
After the Pax Romana, Christianity was the official state religion.
Wherever the Romans dominated, they established Christianity.
Here you go, Teacher should lay off.