Hey there, mate. Here is your answer for this question.
Answer: One important reason for the spread of Christianity was the evangelical missions of Paul of Tarsus. Paul, originally called Saul, was perhaps the one most responsible for the spread of Christianity throughout the Roman Empire. Paul was not one of the original 12 Apostles of Jesus Christ, whose life and teachings are the basis of the Christian religion.
So your answer would have to be A. the apostle Paul
Hope this helps.
