and the face that it's the only pre-colonial Christian church is fascinating.

Well, they weren't really the only pre-colonial Christian church in Africa. Egypt was influenced by Christianity fairly soon after the crucifixion, and Coptic Christians continued after the Islamic conquest in the mid 7th century, to be a majority of the population of Egypt until the 10th century.

Similarly, the Nubian kingdoms of Makuria and Alodia/Alwa in what is now Sudan and South Sudan were Christian kingdoms beginning around the mid 4th century until the mid-14th century after the Mamelukes of Egypt conquered and initiated the process of Islamizing the area.

So, in the mid-4th century the Kingdom of Aksum was part of this trend of the spread of Christianity, and converted like the Nubian neighbors to the north.

The difference is that Christianity survived as the religion of the majority in Ethiopia up to the present century. This happens for a few reasons, a major one is geography. In the era immediately after the Hegira, Axum was a major naval power in the Red Sea, and was responsible for raiding the port of Jeddah. In 702, a Muslim fleet retaliated and destroyed the Aksumite port of Adulis, and established a penal colony of Muslims on the Dahlak archipelago off shore of the former site of Adulis.

Cut off from their major port, and thus from the Red Sea trade that was the source of their wealth, the royal court of Aksum withdrew to the highlands, and Muslim influence spread on the coast of what is now Eritrea. So, throughout much of the middle ages, Christianity (and the Judaisim of the Falashas) survived in the highlands.

A second factor in the survival of Christianity in Ethiopia was that country was in some ways "protected" from any invasion coming from Egypt by their Nubian neighbors. Arab armies had tried to conquer Makuria in the late 7th century, but had been repulsed. After the initial failure, a treaty called the Baqt was negotiated, which ensured peace, free passage of traders between Egypt and Makuria, and the exchange of gifts (Nubian slaves for Egyptian horses).

For one reason or another, the Abbassid (8th and 9th century) Fatimid (10th and 11th century) and Ayyubid (12th and 13th century) leaders of Egypt chose to maintain the Baqt, and keep peace with Makuria. It was only with the revolt of the Mameluke slave-soldiers in the 13th century that there was leadership interested in conquering the Christians to the south. So, Ethiopia indirectly benefited from the Baqt.

Of course, there was a period when Ethiopia being conquered by Muslim armies was a very real possibility. This happened during the Ethiopia-Adal war, when the Somali speaking armies of the Sultanate of Adal (centered around what is now Djibouti), aided by Ottoman soldiers armed with cannon and arquebuses, nearly conquered the Ethiopian kingdom circa 1530. However, the intervention of Portuguese forces on the side of Ethiopia averted complete collapse, and eventually the tide turned and by 1550 Adal had collapsed as a sultanate.

Edit- corrected dates for Adal supremacy and collapse.

Ethiopia is in many ways part of the Middle East, most significantly in that parts of what's now Ethiopia are mentioned quite a few times throughout the Bible.

The Ethiopian Orthodox trace the founding of their church back to St. Mark the Evangelist (this is a traditional claim, not a historical one, as far as I know), but a much older tradition says that much of what's now Ethiopia was called Cush, a nation named for [the grandson of Noah](http://en.wikipedia.org/wiki/Cush_(Bible)). Cush is (among other appearances) one of the places that Isaiah mentions as being wiped out by the Assyrians as they're on their way to conquer Israel. (This is Isaiah telling his rulers "I told you so!" because they'd been trusting the Egyptians and Ethiopians to keep the Assyrians in check - they were major players in the region, in other words.)

There was also Sheba, as in "the Queen of...," which Josephus placed in Ethiopia, and some other medieval historians placed in Eritrea. ("Sheba" might also refer to the seat of the Sabean empire which was in Yemen, where Arabia almost but not quite touches the Horn of Africa.)

In the New Testament (and suitable enough for Good Friday), you might have heard of Simon of Cyrene, the fellow who the gospels say picked up Jesus' cross when he stumbled on his way to the crucifixion. Cyrene was in what's now Libya, but there's some suggestion he was following a well-traveled road of pilgrimage that connected northern Africa (where there was an established Jewish population) with the Jewish Temple (and very new Christian community) in Judah. Acts 8 has the story of St. Philip converting an Ethiopian to Christianity.

Note in that story that the Ethiopian is just kinda hangin' out by the side of the road, reading Isaiah. As you do.

So as for how long Christianity has been in Ethiopia, it's possible to say: nearly all the way back. Within the lifetime of the first Christians, anyway.