The Mediterranean Sea did more to spread religion across the ancient world than any army. Phoenician sailors carried the cult of their own city gods, Melqart of Tyre chief among them, to colonies at Carthage, Cadiz and across Sicily and Sardinia, seeding shrines wherever their ships put in centuries before Rome existed. Greek colonists did the same eastward and westward at once, planting sanctuaries to Apollo, Artemis and Demeter from Massalia to Cyrene, so that a common religious vocabulary ran the length of the sea long before any single power controlled it.
Rome's own conquest turned that patchwork into what its writers called Mare Nostrum, our sea, and the peace it enforced let religious traffic move faster than ever. Jewish communities settled in every major port, Alexandria and Rome above all, carrying their scriptures and their synagogues with them. When Christianity emerged from that same diaspora world, it used the same Roman shipping lanes and the same coastal roads to spread from Antioch and Jerusalem to Rome within a single generation, the apostle Paul crossing the sea himself on mission after mission.
Islam's seventh and eighth century conquests closed the circle from the other direction, bringing the sea's entire southern and eastern rim, Egypt, the Levant and North Africa, under a single new faith within decades. From the Phoenician shrine to the Pauline mission to the Islamic conquest, the same body of water carried each wave of belief to the next shore before the last one had finished arriving.