The basic similarity was that in both Ethiopia and Europe the spread of Christianity was largely a top-down effort. The court in Ethiopia was converted by the emissary Frumentius of Tyre in the early 4th century. This followed by only a few years the similar establishment of Christianity as the official religion of Armenia, making Ethiopia the second country to declare Christianity its state religion. In central and western Europe over 400 years later, the establishment of the Carolingian Empire coincided with a sweeping conversion of the Germanic peoples to Christianity, largely through Charlemagne's military campaigns. But it was a two-pronged effort given that Charlemagne worked in tandem with the Pope in Rome. Therefore though it was a process originating from above, as in Ethiopia, it was directed partly from an outside central authority and not simply confined to an existing self-contained political entity. Rather, the Western Christianizing process occurred as part of a simultaneous political effort uniting previously separate sub-national groups (Franks, Saxons and other Germanic groups) into the Frankish Empire, creating a new realm intended in some sense as a rebirth of the old Roman Empire in the West.
Apart from these factors, the chief difference between the Ethiopian and Carolingian conversions involves the development of Christianity overall at the time these respective processes took place. Ethiopia was christianized before the great ecumenical councils had been held. Like the other Eastern (or more specifically, "Oriental Orthodox") Churches, Ethiopia is geographically relatively close to the birthplace of Christianity, and its connection to the early proselytizing efforts by Christians is a relatively unmediated one. The Ethiopian Church, like the Coptic and Armenian Churches, is considered a "non-Chalcedonian" denomination because it was uninfluenced by the christological formulations of the Chalcedon Council in 451 CE. From today's more ecumenical perspective, this fact might seem insignificant, but for most of the history of Christianity it has meant that these Eastern churches were considered outside the True Church by not only Rome, but by Constantinople as well.