Eritrean Christianity shares its origin with the Ethiopian church, both tracing to the Aksumite kingdom's fourth century conversion under Frumentius and to the Nine Saints who founded monasticism in the region in the following century. For most of that shared history the highlands that became Eritrea and the highlands that became Ethiopia fell under a single church, itself subject to the Coptic Patriarch of Alexandria until 1959 and then to the newly autocephalous Ethiopian Patriarchate.
Italian colonization of Eritrea from the late nineteenth century, followed by British administration and then, from 1952, federation with and eventual annexation by Ethiopia, gave Eritrean Christians a distinct political history even while their church remained administratively Ethiopian. A thirty year war of independence, concluded in 1991 with Eritrean secession and formalized in 1993, created a new state whose church, still under the Ethiopian Patriarchate, no longer matched the new political map.
Ethiopian and Coptic authorities recognized the resulting anomaly. In 1993 Pope Shenouda III of Alexandria, the senior Oriental Orthodox see, granted the Eritrean church its own autocephaly, and in 1998 it was elevated to a Patriarchate in its own right, completing a separation that mirrored the church's shared origin with a genuinely separate modern institution.