Answer:
While Christianity is mostly a Middle-Eastern and hence an 'Asian' religion, it grew very quickly in Europe when it was adopted by the Roman Empire.
The Roman Empire spanned the whole of Europe and parts of North Africa and Middle East. As the Empire adopted the new religion, it was easily the largest Christian Empire in the world with the centre at Rome.
As the Empire receded, Rome emerged as the center of Christendom in the West, while in the East, the remains of the Roman Empire, centered around Constantinople, became the religious Centre in Asia.
Gradually, both took a different path, with Roman emerging as a Catholic power while Constantinople began a Greek orthodoxy.