There are similarities in the fragmentary stories of the development of Christianity in Asia and Eastern Africa during the fourth to sixth centuries. As it had from the beginning, it followed the trade routes as merchants and missionaries took with them their faith. There was a Christian presence in Edessa (ancient and modern Urfa) from the earliest days of Christianity. This chapter first discusses three primary theories of the development of Christianity in this region: the Thomas traditions; the Abgar-Addai traditions; and Jewish origins. Next, it explores the evidence of Christianity in northern Mesopotamia during the second, third and fourth centuries. The diffusion of Christianity throughout Persia was caused by various factors including adaptation of Christian ideas within existing communities (which may be related to trade patterns). The chapter focuses on the major geographical, political or ethnic factors under which Christianity developed in Adiabene, Armenia, Georgia, India, Egypt (Coptic Christians), Nubia, Ethiopia, South Arabia, Soqotra, Central Asia and China.