See below for a map on the spread of Christianity through 300 AD to 600 AD.
More ABout the SPread of CHristianity
Christianity spread throughout Europe because it was the official language of Rome. The monks helped spread Christianity too. A monk, as stated from Google definitions, is "a member of a religious community of men typically living under vows of poverty, chastity, and obedience." A missionary is a person sent on a religion mission. The missionaries, after the fall of the Roman empire, spread Christianity throughout Europe, gaining much power for the Roman Catholic Church.