They could of come to some basic conception of God through speculative philosophy like Plato and Aristotle, everything in the universe is contingent upon something else so what was the force that began motion, to say that one part of nature is God and the other isn't doesn't make sense, if the God's are divided then why hasn't their conflict ended, why can't the good God conquer the evil God, or why they could of known God through the moral law and it's ethics, it's painfully obviouse that you shouldn't enslave innocent people and there's nothing in nature to say that you should do it, so they could of learnt like that. to tell you the truth i know nothing about Aztec mythology but I'm Gona guess and say it was a polytheistic religion, theirs God's were likely at one point the only God of a certain tribe but we're grafted in to the pantheon by conquest or just the mixing of tribes, their God like every other man made God would of had flaws like being corruptible or susceptible to bad moods, Plato saw in this that the Greek God's were always just works of fiction and reflections of men so they could of learnt that way.
If God had revealed himself to the Aztecs they were worse than Sodom and Gomorrah so I'd say they got off lightly being in the dark, if lets say the disciples could of gotten to the Aztecs or Indians sooner they would of had a worse reception than they did in jerusalem, they would not of been able to communicate first off, they had no similar beliefs so they could not relate or compare anything with their culture or religious views, it would of been awful. Saying that though, according to the Jewish law you didn't need to know God to be saved, salvation was dependant upon the law and the moral law is written on the hearts of man, so who knows how many good people were saved amongst then
A very good book that's sort of on this is g.k Chesterton everlasting man btw