Axum was the center of the marine trading power known as the Aksumite Empire, which predated the earliest mentions in Roman-era writings. Around 356 CE, its ruler was converted to Christianity by Frumentius. Later, under the reign of Emperor Kaleb, Axum was a quasi-ally of Byzantium against the Sasanian Empire which had adopted Zoroastrianism. The historical record is unclear, with ancient church records the primary contemporary sources.