The armies as such had very little interest in that, and the Aztecs even less.
By 1531, after several decades of Spanish domination there were very few -- perhaps a hundred or less -- indigenous Catholic converts in Mexico. But by 20 years after that, they numbered in the millions.
What made the difference? Nuestra Señora de Guadalupe.
In the book ‘The Broken Spears: The Aztec Account of the Conquest of Mexico’, it is described that the Aztec leaders of many areas were insisting? (pleading?) to be converted to Christianity, and that Cortez had to insist/explain, that they learn something about what they were doing first, that they needed to understand what accepting Christ meant.
“What made the difference? Nuestra Señora de Guadalupe.”
Someone should have told the Bishop of Mexico. He didn’t seem to have heard anything about Juan Diego, the apparition, or any miraculous mass conversion of the Indians. In fact, it wasn’t until over a hundred years later that we have any evidence of that story.
Horny Conquistadores breeding with the natives.
Those are your modern-day Mexicans.