Christianity has its origins in Roman-occupied Judaea, and became a global cultural force largely because of the Roman Empire. Constantine the Great made Christianity legal in the Roman Empire, and he also convened the First Council of Nicaea to standardise Christian doctrine and dogma for the first time.

However, almost simultaneously, Christianity was becoming the official religion of another great empire as well. Ezana of Aksum (r.320s-360) is generally considered the first Ethiopian monarch to embrace Christianity, and he was a contemporary of Constantine (whose reign as one of four co-emperors began in 306, and was the sole emperor from 324-337). By the time Ezana took the throne, Aksum had expanded from its territorial core in modern-day Eritrea and northern Ethiopia, and had conquered much of south Arabia, too -- including the Jewish tribes in Himyar. Under Ezana, Aksum would expand northwards, and conquer the Kingdom of Kush.

What would have happened if Rome had not been Christianised under Constantine, but Ethiopia had been Christianised under Ezana? What affect would a Christianity which looks to Aksum rather than Constantinople have on the world?

First of all -- I don't think Christianity would stop being a cultural force within the Roman Empire. Christianity was a very widespread and popular movement, and it would probably be legalised eventually. This scenario seeks to explore what would happen if Christianity hadn't been legalised in Rome until a generation or so after it had been been adopted in Aksum.

Now, needless to say -- although Aksum was a powerful and wealthy realm, it was not equal to Rome. Aksum might have the influence to spread Christianity across the Horn of Africa, the Arabian Peninsula, maybe even somewhat further beyond across the Indian Ocean, but I think it's doubtful that Christianity would spread nearly as quickly without the institutional support of the Roman Empire. Christianity certainly wouldn't reach Northern Europe without Rome, for example. I imagine that Egypt, Palestine, and Syria might remain centres of Christianity within the Roman Empire -- in part because of their religious significance to Christians, but also in part because of their proximity to Ethiopia and Arabia, and the influence of Christian merchants and pilgrims from those areas.

However, I also think Christianity would probably be a lot more anti-Roman in its character. A Christian Ethiopia would likely become a homeland for early Christian refugees from Rome, who'd probably resent having been forced to flee. I don't know if Christianity would still have its normative anti-Semitic bent (after all, Jews still reject Jesus as the messiah), but it's probable that Christian doctrine would blame the Romans for Jesus's death, rather than the Jews, since the story of Jesus's crucifixion would be reinterpreted by non-Romans and by people forced to flee the Roman Empire, and not by Roman institutions seeking to legitimise themselves. Aside from that, Aksum had a large population of Jewish subjects in Himyar and elsewhere, and many of these Jews were also the descendants of refugees who also fled the Jewish Revolt and the destruction of the Temple. Perhaps Christianity would take a more conciliatory attitude towards the Jews, and a harder stance against the Romans, if the Roman Empire had not embraced Christianity when it did.

So, what do you think?
Cheers all!
From what I understand, Ethiopian Christians still follow more of our dietary taboos than most Christians. So you may see Jewish dietary law retained, especially given, as you note, the high number of Jews in Aksum.
In all honesty this would require a second soft PoD of Ethiopia being more thoroughly Christianized. Under Ezana and his successors Christianity in Ethiopia was pretty much limited to the ports and the city of Axum itself. It was only under St. Kaleb that Christianity really spread to the rest of Ethiopia. So you'd need Christianity to spread through Ethiopia more thoroughly.

But nevertheless we'd see a much more different Christian world. A lot of traditional Jewish practices like circumcision and dietary restrictions would be maintained, as would the practice of keeping Tabots within churches. And if we're looking later down the line this would mean Miaphysitism would also be dominant over mainstream Diophysite Christianity.

Aside from the Christian practices, I could also see the Roman faith schism from Ethiopian Christianity as well, since early church scholars disagreed with many of the "Judaizing" practices that formed the heart of the Ethiopian Church. So you could have two distinct churches with independent structures, and a firm animosity for one another. The Ethiopian rite would also become more popular, as it would spread across the Middle East and Africa with state sponsorship from Axum and other nations and tribes, while the Roman rite is forced into slow growth with a possible resurgence in persecution.