Christianity. The Roman Empire became Christian, and this included present day Syria, Jordan, Palestine, Turkey, Egypt, etc. So, all of these areas used to have many, many churches, and played a large role in the further formation of Christianity. Remember the Hagia Sophia was built as a Christian Church, and there are many Saints from this area and era as well, such as St. Simeon from near Aleppo, Syria.

The Crusades are seen as Christian aggression, but remember, Christian leaders traveled from the middle east (their home) to seek out help from the west against invaders. (Not trying to gloss over violence, but the original aim of the Crusades was not "let's go conquer new territory", but rather: "let's go rescue fellow Christians / take back territory". )