The only issue with that is the church itself. The Catholic Church preferred Western Europe divided because it gave the pope more power and influence. If you have a united western Roman Empire again the pope knows he could end up becoming second to the emperor like the patriarch of the Eastern Church. Petty kings and nobles can’t push back against the pope like a emperor can. That’s one of the reasons the Holy Roman Empire was more of a confederation then empire.
This idea can also work the other way around, however. Extend the Pope's temporal power to such an extent that he essentially
is the Emperor. I stress the relevancy of Christianity because religion is the most powerful cohesive agent I'm aware of-- it binds peoples together, gives them a common structure (both in the socio-political world and in their intellectual and cultural premises). Nothing else works quite as well. Nationalism is exclusive by definition, it always divides to some extent. And Empires... they have their own baked-in deficiencies, which I'll get into below.
The idea of being Roman on the other hand seems comparable to the Chinese one. For example, cantonese and mandarin would be considered different cultures and languages in most other places but they are both considered Chinese. Could you have something similar develop among Romance or Hellenistic people?
I don't think it was truly similar to the Chinese sense of cohesion, in that the Romand
lacked enough of a cultural commonality to survive long-term political fracturing. If your Empire falls apart and its cultures soon cease to identify as "Roman", then there is already something lacking (compared to the Chinese situation). It has often been noted that the c. 500-year Han dynasty has remarkable similarities to the c. 500-year (Western) Roman Empire, right down to the division into two distinct periods with a period of crisis in between. So why did China "fall back together again", while Rome didn't? I hardly think that blaming the one element that created unity in the post-Roman world makes logical sense. On the contrary: I think Rome spent too long, and expended too much energy, on attempting to cling to its dying traditions and resisting Christianity.
Allow me to venture that if Rome had whole-heartedly embraced Christianity much sooner, and simply transmuted its belief in the divinity of the Emperor in a notion that the Emperor was divinely ordained by God... then we might have seen a Christian Rome "falling back together", too, in the end. In such a scenario, the religion and the empire would have been more intrinsically tied together. Christianity would have been more of a "truly Roman thing", and the figure of the Emperor would have been more central and essential to a Christian understanding of the proper world-order.
The issue I see with Christianity is it undermines the ideas rooted in Roman culture and identity. In Rome your supposed to serve and be loyal to the empire above all else. They seem almost proto nationalist or proto fascist in some of their belief. I feel like a dogmatic religion like Christianity undermines that due to religion being valued over complete loyalty and service to the empire.
I do not think this is correct. Rather, the pre-existing beliefs were already losing their support. They were held onto in a local fashion (almost all religion was local), but Christianity was only so successful because it offered something that people
craved. The story that Rome had become religiously sterile is not fully true at all, but religion was bound to
change. As of the first century AD, we really see a lot of innovative cultism kick off and gain traction. Why? Because there was a "market" for such things.
In the end, the old Religio Romana wasn't the true competitor for Christianity. Other cults and ways of thinking were on the rise as well. If "old religion" had survived, it would have been by adopting a very "Christian-like" organisational structure, and a lot of the same philosophical influences. Think of the stuff Julian the Apostate had in mind, or of Manichaeism, for instance.
So a big change was always coming. Taking out hristianity only changes who gets to do the changing... but things won't stay as they were.
Let me also note that the Romans weren't at all proto-nationalist during their Imperial phase. They were Imperialist, which is another thing. Empires aren't nations. Moreso than nations, Empires are abstractions. Loyalty to an Empire isn't an impulse, like loyalty to your kin. In many ways, every Empire is a great myth that only exists as long as people believe in it. A myth is strongest when you can combine the political with the sacred. The Mandate of Heaven accomplishes this. Rome never really did. The attempts to deify the emperor didn't have the same status at all, muddled and heterodox as the religious realities of the Roman Empire were. But Christianity offered such status, and offfered an organised structure of unifying religious commonality.
If the Emperors had, as soon as feasible, proclaimed themselves as divinely ordained by God, and had made Christianity their state religion... that could have offered an idea with lasting power. Strong enough to survive political fragmentations and inspire re-unifications. Their very own Mandate of Heaven.
Civil duty and values came into conflict with Christian ones. Christian Romans literally helped invading Christian barbarians over Pagan Roman citizens. It likely isn’t a coincidence the empire fell less then a hundred years after adopting Christianity. Without Christianity, could the idea of “Rome” stay stronger especially if the empire last longer?
The answer to my question is "probably not", as should be clear by now. The thing is that it wasn't Christianity that killed Rome. I've often argued that stable Empires fall a few centuries after they stop expanding. The adoption of Christianity is largely unrelated. Considering that Christianity was successful for a
reason, preventing Christianity (or any alternative to it) from arising would only leave people in a religious environment that no longer sufficiently inspires or captivates them. Would that inspire re-unification of Rome later on? Not at all.
To recapitulate: I don't think Rome made a mistake by adopting Christianity. I think Rome made a mistake by resisting Christianity for too long. Embracing it wholeheartedly at an early stage would've changed very little about the fall of Rome, but it would've made a "China-like" resurrection of Rome considerably more viable.