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.
The christian religions is not like the Catholic who has a Pope as it's leader. There are too many different types of Christians to have one person as a head. Each type of the different types of churches has their own type of governance and leaders.
Technically, Jesus is responsbile for the origin of Christianity, seeing as the religion is based around him. However, the tradition of Christianity was not set by Jesus; it developed over time, but was begun mainly by early Jewish Christian believers, especially the twelve apostles, who functioned as early leaders. Saint Paul (and later, more extremely, Marcion) are largely responsible for separating Christianity from Judaism.
Paul did the most to found Christianity, even if he imagined that he was seeking to broaden the appeal of Judaism beyond the Jewish people - to non-Jews (Gentiles). He wrote more of the New Testament than any other author, with numerous letters (or Epistles).
The Emperor Constantine, by sanctioning Christianity as the official religion of the Roman Empire, probably did the most to legitimize Christianity as a major religion in the world.
Many people believe that Jesus leads Christianity, even when he's dead.
The pope.
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.
Patrick is responsible for converting much of Ireland to Christianity.
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