Cosmas Indicopleustes

[5][6] Around AD 550, while a monk in the retirement of a Sinai cloister,[7] Cosmas wrote the once-copiously illustrated Christian Topography, a work partly based on his personal experiences as a merchant on the Red Sea and Indian Ocean in the early 6th century.

[8] While it is known from classical literature, especially the Periplus Maris Erythraei, that there had been trade between the Roman Empire and India from the first century BC onwards, Cosmas's report is one of the few from individuals who had actually made the journey.

[…] The island [of Sri Lanka] has also a church of Persian Christians who have settled there, and a Presbyter who is appointed from Persia, and a Deacon and a complete ecclesiastical ritual.

"[9]A major feature of his Christian Topography is his worldview that the surface of ocean and earth is flat (that is, nonconvex and nonspherical, as perceived by the human senses) and that the heavens form the shape of a box with a curved lid.

[2] David C. Lindberg asserts: Cosmas was not particularly influential in Byzantium, but he is important for us because he has been commonly used to buttress the claim that all (or most) medieval people believed they lived on a flat earth.

In the book, a Byzantine priest and spy, Zozimas of Chalcedon, refers to his world topography as the key to finding the mythical Prester John: Well, in the empire of us Romans, centuries ago there lived a great sage, Cosmas Indicopleustes, who traveled to the very confines of the world, and in his Christian Topography demonstrated in irrefutable fashion that the earth truly is in the form of a tabernacle, and that only thus can we explain the most obscure phenomena.

On request of the Axumite king and in preparation for this campaign, he recorded now-vanished inscriptions such as the Monumentum Adulitanum which he mistook for a continuation of another monument detailing Ptolemy III Euergetes's conquests in Asia.