The Catholic Church did NOT "help" to spread Christianity. It did spread Christianity through the lives and witnesses of thousands of saints over the centuries. The Catholic Church is the only Church founded by Our Blessed Lord, and He guaranteed to be with it until the end of the world. (Matthew 28:20)
The vassals (knights) were Christian so that led to it being spread.
AnswerAll of the popes have made it a fundamental objective to spread Christianity.
Kings and Monasteries helped to spread Christianity.
Paul of Tarsus, who was previously known as Saul spread Christianity as the first Christian missionary, or person who spreads their belief to non-believers. He travelled across the Roman Empire, sending Christian messages to gentiles and non-Jewish communities as well as to Jews. This allowed more people to follow the faith. He preached about Christianity in new areas where no one had preached before, and established Christianity there.
Paul
clovis helped spread christianity by making it the only legal religion
Kings and Monasteries helped to spread Christianity.
Charlemagne helped spread Christianity throughout the Frankish Empire and the lands he conquered. He spread Christianity through both peaceful conversion efforts and military conquests.
medieval missionaries
the bible
They built the Vatican.
Trade.
As Christianity spread it faced many oppositions. Some of the Christian that helped in spreading the gospel were persecuted, jailed and tortured.