The history of Ethiopia in the Middle Ages roughly spans the period from the decline of the Kingdom of Aksum in the 7th century to the Oromo migrations beginning in the mid-16th century. Aksum had been a powerful empire during late antiquity, appearing in the Periplus of the Erythraean Sea and mentioned by Iranian prophet Mani as one of the "four great kingdoms on earth", along with the Sasanian Empire of Persia, the Roman Empire, and China's Three Kingdoms. The kingdom was an integral part of the trade route between Rome and the Indian subcontinent, had substantial cultural ties to the Greco-Roman world, and was a very early adopter of Christianity under Ezana of Aksum in the mid-4th century. The use of "Ethiopia" to refer to the region dates back to the 4th century. At its height, the ki