Answer:
Used to be Christianity was spread through word of mouth to spiritually save another. Later many people were forced to be converted through fear such as Native Americans. But now it still retains the tradition of word of mouth, missionaries and even television advertisements.
Answer
In the very beginning, starting with Rome, Christians weren't popular at all. But once they started getting persecuted they became famous. It became a "cooler" religion with the underground church meetings and stuff. It spread along the Silk roads with missionaries and etc. after that.