It began to spread when Jesus was ministring, although His followers weren't called Christians until much later. Perhaps the biggest single event that caused massive conversions was the day of Pentecost, when 3,000 people turned their lives over to Christ.
Christianity is the united earthwide congregation of those who follow and TEACH the true teachings of Jesus Christ, so technically it started to spread when Jesus started to preach, but John the Baptist prepared the way for Christianity. For six months-from the spring to the fall of 29 C.E., he focused the Jews' attention on the coming Messiah and preached about the coming Messiah among the Jews in the Roman Empire, so you COULD say it started a little earlier.
When Jesus began his ministry, people who had been following John, were directed to now follow Jesus. (John 1:35-40)
In 29 C.E. Jesus was anointed with holy spirit and became the promised Messiah, or Christ. (Gal. 3:16) Jesus would spend the next three and a half years "spreading Christianity" by preaching the good news of the Kingdom to the Jewish nation under Roman rule. After his death, his disciples continued the preaching work to the most distant parts of the earth.
That is a wide question with many possible answers. However in simplest terms the largest spread of Christianity into the Roman empire was due to the fact that Roman Emperor Constantine I, also known as Constantine the Great, legalized Christianity and made it the state religion of Rome, after what he claimed was a vision from God before a battle. He didn't actually convert to Christianity though until he was on his death bed.
Christianity grew and spread in the Roman empire from about 35 AD (this is a loose estimate). Christianity began in the Roman empire, remember that Judaea was part of the empire and it was the birthplace of Christianity. From Judaea it spread by way of the missionary work of the disciples of Jesus.
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
The spread of Christianity had political ramifications for the Roman Empire. The empire split into two, Constantinople to the east and Rome to the west
No religion helped to save Roman culture. One religion spread through the Roman Empire in its later days and transformed Roman culture: Christianity.
It could not decline from nothing - it did not exist until after much of what was to become the Roman empire was already under Roman rule. It spread, particularly after the Romans accepted Christianity under Constantine
Which Roman leader exactly? Whom are your referring to?
Christianity did not easily spread through the Roman Empire. See the related question, "How did Christianity spread throughout the Roman Empire?"
The Roman Empire.
That would be Christianity.
By 325ad Christianity had spread throughout the Roman Empire and to Britain. If you google "spread of Christianity map" you will find maps showing the spread.
Christianity
They used force
The spread of Christianity had political ramifications for the Roman Empire. The empire split into two, Constantinople to the east and Rome to the west
No religion helped to save Roman culture. One religion spread through the Roman Empire in its later days and transformed Roman culture: Christianity.
It could not decline from nothing - it did not exist until after much of what was to become the Roman empire was already under Roman rule. It spread, particularly after the Romans accepted Christianity under Constantine
Christianity was certainly a constant. Christianity was also spread beyond the area of the former western part of the Roman Empire with the conversion of previously pagan peoples.
The Romans who remained pagans did not contribute to the spread of Christianity Neither did Julian, who was the only pagan emperor in the Later Roman Empire
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