It is often said the Crusades, which killed millions of Muslims, were a noble undertaking by the Vatican because without them we would now be Muslims. We don’t know this and I have my doubts.
When Europeans came to the Americas there were civilizations here that were, to say the least, noteworthy. The Incas and Aztecs, among others, were advanced in mathematics and astronomy. Their organizational skills were exceptionally well developed. They bred animals and crops. They did not have the weapons Europeans had but might have been more successful against the European invasion but for one misfortune, disease.
Europeans, because of their diversity, had resistance to diseases than the more isolated natives in the Americans. Something like 40% of the native populations of South, Central and North America died. People dying plus those caring for the sick spelled defeat.
We can talk theoretically about conversions. That is, could the Europeans have converted Indians to Christianity without the force of the gun? We’ll never know, but we do know the marketing program of Christianity was aimed at pagans, not at those natives of the Americans.
Plus, American natives were not under the thumb of Rome. Europeans could celebrate release from a conqueror. I doubt natives would have celebrated being conquered.
I would guess many Christians would see the spread of Christianity across the American continents as inevitable. Probably these same folks see its future to be as successful as its past.
It is apparent today that neither its past nor its future are inevitable successes.