For its first couple of hundred years, the surviving disciples and new disciples who were appointed as bishops managed the emerging Christianity religion and way of life.
In 313 AD, the Edict of Milan made the church legal and accepted in the Roman Empire. A few years later the Council of Nicaea established a more formal structure for the church which evolved into the Eastern Orthodox and Roman Catholic branches of the church. The Bishop of Alexandria was then in charge of the Eastern Church and the Bishop of Rome (Pope) was in charge of the west.
No culture is 'responsible' for Christianity. Christianity is based on the death, burial and resurrection of Jesus Christ.
The apostle responsible for spreading Christianity among the Jews was Paul.
Many cultures are responsible for the spreading of Christianity. The main culture which spread it was certainly the Roman Empire.
Thomas was responsible to bring Christianity to India.
He is not responsible for either.
It was the apostle Paul .
The apostle Paul spread Christianity throughout the Mediterranean regionSaint Patrick spread Christianity throughout Ireland!
He preached Christianity to the heathens in Ireland.
The emperor Constantine.
Paul is probably responsible for the spread of Christianity, after Jesus himself, of course. But without Paul or someone like him, Christianity may have remained an obscure cult of Judaism.
Monks and missionaries
AnswerArguably, there are two persons who, more than any one else, were responsible for the spread of Christianity. The first of these was Paul, the "Apostle to the Gentiles", without whom Christianity could have remained a minor Jewish sect. The second is the fourth-century Emperor Cosntantine, who gave the Christian Church state patronage and began the process of converting the empire to Christianity.