The Saint Thomas Christians of Kerala hold that the apostle Thomas landed on the Malabar coast in 52 and founded churches there before dying at Mylapore. The tradition is old and the community is unquestionably ancient, with documented links to the Church of the East in Persia from at least the fourth century; the apostolic voyage itself rests on tradition and the apocryphal Acts of Thomas rather than on contemporary record, and historians differ on it. Either way, the community is evidence that Christianity reached India by sea long before it reached most of Europe by land.
Pilgrimage was the ocean's other great traffic. Hajjis from Southeast Asia, India and East Africa sailed to Jeddah on the same monsoon schedule as the cargo, often spending a year or more away, and the pilgrim ships of the nineteenth century carried tens of thousands a season. The scholarly networks that resulted, students from Java and Sumatra studying in Mecca and returning as teachers, reshaped Islam in Southeast Asia repeatedly.
The coasts are lined with the results. Sufi shrines and saints' tombs stand at ports around the whole rim, from the Swahili towns to Gujarat to Aceh, serving pilgrims and, in many places, sailors making vows before a voyage. The ocean also carried a slave trade, from East Africa to Arabia, Persia and India, that ran for over a millennium and left communities of African descent around its northern shores.