Huh? Christianity and Islam both have billions of followers? OP are you trolling?
DBAHC stands for double blind alternate history challenge. Basically in a double blind (DB) scenario it means what the op posted is real and where discussing how to change that.

E.g. DBWI the Nazis lost. The question is inferring we live in a world we're the Nazis won and asking how would the world look like if they didn't.
 
But anyway, back on topic. What would you change about Christianity to give it more appeal among the Romans?

Keeping their Christ figure and basic theology intact? Basically ASB. If they're still unified around the time of Islam now... that's viable if translation of the Koran into Latin is allowed. Muhammad has an ethos well in line with Roman temperment at their height, and the Emperor's title of semidivinity could seem into the title and powers of the Kalaph.
 
The way I see it, Christianity and Islam both suffered different problems.

Islam's problem was a precipitous drop in its perceived moral authority early on due to the horrific infighting. This, combined with the fact that Kaliphate had made a whole lot of enemies both inside and outside its territory, meant that the Muslims couldn't follow up on their initial successes, and so the Near Eastern polytheists pretty much eroded most of their gains. Which explains why, nowadays, you're far more likely to find devotees to Horus, Dagon, Yahweh, Baal, Enlil, or even Hubal than Allah in most of the Middle East. It also explains why so many Muslims abandoned the idea that Allah was the only deity.

Christianity's problem, on the other hand, was a lack of appeal among the peoples of the area. Even in Judea, its reception wasn't particularly positive despite a seemingly promising start. Might Christianity have found a more receptive audience elsewhere?

EDIT: Sorry, I accidentally hit post before I was finished typing.
 
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