From the time of Christ until the fall of the Roman Empire 313 A.D., the missionary zeal of ordinary people, particularly those who traveled - soldiers, traders, muleteers and diplomats - spread the persecuted faith throughout the far corners of the Empire – a land encircling the Mediterranean or Middle Sea. The Church established itself as a state-within-a-state performing acts such as aiding the needy, educating the children, and adjudicating disputes.

http://www.encyclopedia.com/html/section/Constnt1Rom_RisetoPower.asp

In 313, a Roman claimant to the throne of the Julii saw in the sky a flaming cross and the words “by this sign you will conquer.” This Roman, Constantine, embraced Christianity, emblazoned the Chi Rho – the Greek symbol of Christ on the shields of his armies, conquered his enemies and became Emperor. Constantine declared Christianity the state religion.

http://www.newadvent.org/cathen/02084a.htm
http://www.newadvent.org/cathen/08341a.htm

Men like St Jerome (340-420) who spent most of his life in a monastery in modern Syria and author of City of God, St Augustine (354-430), a Northern African from Hippo on the outskirts of Carthage located in modern day Bizerta, Tunisia shaped Christianity throughout the modern age.

http://www.newadvent.org/cathen/11549a.htm

By 451 A.D., the state religion of the new Roman Empire had patriarchs, Jerusalem, Antioch, Alexandria, Constantinople and Rome, with Rome having primacy.. Today, though Jerusalem is under Jewish control, three of the five cities – (Antioch in Syria, Alexandria in Egypt and Constantinople renamed Istanbul in Turkey) of the patriarchate remain in the hands of the Muslim.