Cosmas Indicopleustes (Koinē Greek: Κοσμᾶς Ἰνδικοπλεύστης, lit. 'Cosmas who sailed to India'; also known as Cosmas the Monk) was a merchant and later hermit from Alexandria in Egypt. He was a 6th-century traveller who made several voyages to India during the reign of emperor Justinian. His work Christian Topography contained some of the earliest and most famous world maps. Cosmas was a pupil of the East Syriac Patriarch Aba I and was himself a follower of the Church of the East.
Around AD 550, while a monk in the retirement of a Sinai cloister, 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. His description of India and Ceylon during the 6th century is invaluable to historians. Cosmas seems to have personally visited the Kingdom of Axum in modern day northern Ethiopia, as well as Eritrea. He sailed along the coast of Socotra, but it cannot be ascertained that he really visited India and Ceylon.
"Indicopleustes" means "Indian voyager", from πλέω "(I) sail". 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. He described and sketched some of what he saw in his Topography. Some of these have been copied into the existing manuscripts, the oldest dating to the 9th century. In 522 AD, he mentions several ports of trade on the Malabar Coast (South India). He is the first traveller to mention Soriyani Christians in present-day Kerala in India. He wrote, " Even in Taprobane there is a church of Christians, which clergy and body of believers, but I don’t know whether there are any Christians in the country beyond it. In the country called Maale where the pepper grows, there is also a church, and at another place called Kalliana there is moreover a bishop, who is appointed from Persia. In the island of Dios-Korides which is situated in the same Indian Seas, and where inhabitance speak Greek, having been originally colonists sent by Ptolemies who succeeded Alexander the Macedonian, there are clergy who receive their ordination from Persia, and are sent to the island, and there is also multitude of Christians" 
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. He was scornful of Ptolemy and others who held that the world's surface was, contrary to human perceptual experience, a spherical shape. Cosmas aimed to prove that pre-Christian geographers had been wrong in asserting that the surface of the earth and surface of the ocean was convex and spherical in shape, and that it was in fact modelled on the tabernacle, the house of worship described to Moses by God during the Jewish Exodus from Egypt. In the centre of the plane is the inhabited earth, surrounded by ocean, beyond which lies the paradise of Adam. The sun revolves round a conical mountain to the north: round the summit in summer, round the base in winter, which accounts for the difference in the length of the day.
However, his idea that the surface of the ocean and earth is nonspherical had been a minority view among educated Western opinion since the 3rd century BC. Cosmas's view was never influential even in religious circles; a near-contemporary Christian, John Philoponus, disagreed with him as did many Christian philosophers of the era.
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. This claim...is totally false. Cosmas is, in fact, the only medieval European known to have defended a flat earth cosmology, whereas it is safe to assume that all educated Western Europeans (and almost one hundred percent of educated Byzantines), as well as sailors and travelers, believed in the earth's sphericity.
Cosmas was mentioned in Umberto Eco's historical novel, Baudolino. 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.
Cosmology aside, Cosmas proves to be an interesting and reliable guide, providing a window into a world that has since disappeared. He happened to be in Adulis on the Red Sea Coast of modern Eritrea at the time (c. AD 525) when the King of Axum was preparing a military expedition to attack the Jewish king Dhu Nuwas in Yemen, who had recently been persecuting Christians. 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. Neither have been located by archeologists. Allusions in the Topography suggest that Cosmas was also the author of a larger cosmography, a treatise on the motions of the stars, and commentaries on the Psalms and Canticles.