Traditional African religions are almost all heavily blended with either Christianity or Islam or both, while in Indonesia Islam spread mostly peacefully due to the advantages it offered rulers. In fact, Islam even grew in West Africa during the era of European colonialism. But once again, it's worth noting that we don't have an analogue for the situation Mesoamerica would be dealing with since it is very likely sizable Spanish enclaves would exist along the coast along with Spanish colonies not far away in Central America and Cuba.Traditional religions in Africa and Indonesia still very much exist though. Traditional African religions are still practiced by relatively large groups in many ostensibly Muslim or Christian countries. Consider that Christianity in particular was forced onto the region during the scramble, and that prior to that, while there were Christian states such as the Kingdom of the Congo, most African states did not convert to Christianity. Even in places where contact with Christian European were frequent, the majority religion was pagan until the early 20th century, such as in the case of the Akan religion, or that of the Yoruba. Such states are a much better analogy for this scenario, because there we see the presence of Christianity in the absense of a Christian state apparatus to enforce the mandated religion. You may note, Christianity failed to become the majority religion there by peaceful means and required state coercion to become dominant.
That doesn't change the fact that lower classes still converted to the religion of the elite and attributed aspects of the old gods they worshipped to Christian saints.I think you overestimate the relevance of the upper classes to the lower. Most of what the rulers believe is completely disconnected to what is relevant for their subjects. Medieval Christian peasants did not care very much for the complex theology elaborated by priests and nobles and stubbornly refused to accept unpopular church teachings for centuries. The Catholic Church tried really, really hard to eliminate belief in magic and faeries and witches, and at that they roundly failed, for these beliefs were still very much alive when the reformation came around, and indeed they are still held by sizeable populations throughout Europe and the Americas. Or consider the struggle of the westernized Indian upper class to actually enforce the secular constitution they wrote up. Unless you want to crack down hard against any one particular practice, it isn't going just because the upper class doesn't like it.
Incidentally, at times the lower classes were far more orthodox than the elite, like the radical Protestant movements in the 16th century or during the English Civil War. The Fulani jihads (which would be part of the Islamic tradition of orthodox Muslims overthrowing decadent rulers) and no doubt various Buddhist peasant revolts in China and Japan might count as well.
They're the ones keeping the imperial court happy with huge tribute demands, and the ones who don't like the imperial court to begin with because of said tribute demands. This is thus an instance of where promoting Christianity would benefit the elite since it keeps these people happy.Lets also not forget that the people who are in charge in Tenochtitlan are not the people who would feel compelled to convert. Those are regional rulers and merchants in the east of the empire, distinct from the important nobles of the central highlands. They have absolutely no power over the imperial court - that's why they convert in the first place.
I don't see how it's any different from the role of pagan rulers in Germanic and Slavic Europe where their dynasties traced descent from deities and one of their most important roles was to preside at feasts in honour of the chief god of the city. And clearly plenty of nobles were willing to collaborate with the Spanish since the ruling dynasties of many altepetl still held much power in New Spain, which suggests their power was more based on lineage than religion.The imperial nobility will never convert, under no circumstances, because the Nahua religions works for their ambitions, while Christianity is contrary to them. They had their jobs because of their religion, and if they were Christians they'd loose most of their secular power. The Nahua faith was integral to the Mexica empire. It could no more convert to Christianity than an Islamic caliphate could. If it did, it would almost certainly disintegrate.
The Spanish had plans to colonise Argentina before the conquest of the Inca, and it's also far west of the Tordesillas line so no Catholic nation could legally claim it. Asuncion in modern Paraguay was the first real Spanish center, and it would likely be even more prominent as a center for trading with the Inca (or raiding them no doubt).if i had to guess who would found buenos aires it would be the country that colonized brazil in ttl. the Argentine west will be an area of struggle between colonizers and the Incas. Probably with the colonists winning and the division of the two nations being the Andes
But the Aztecs would primarily have dealings with Catholic powers (Spain, France, maybe pre-Reformation England) for at least a century, it wouldn't benefit them to promote their own Christianity even if they'd be tolerant of the folk Christianity in the countryside. It's like how Kongo was officially Catholic even if the vast majority of people only had some vague ideas about Christianity.What I think is most likely to happen is the new post-Aztec power will have a bastard version of Christianity. Much more than in OTL, to the point of not being considered Catholic (as it is considered in OTL). The Incas I don't know, their religion is something less extreme and is something more intense with the leader having a divine position. As much as no one likes the Inca religion, it is much more acceptable than the Aztec. They may lose territory, but the core of the empire can remain strong. With religion adapting to be more palatable to the world. Sacrifice only with animals for example.
I don't bring up the Inca because they seem more likely by far to keep their religion given their divine monarchy and relative isolation, since the coast would be the most depopulated by European introduced diseases (as happened in lowland Ecuador and parts of Peru) and would likely be most easily seized/purchased by Europeans (I've always loved the idea of Callao as a South American Macau). There's going to be far less pressure and incentive for them to convert, although it's possible if the state declines enough/is colonised.