Pre-Columbus Indians were not murderous, rampaging hordes. They had moral systems that we can easily call admirable. Note that there is no record whatsoever of large armies of Indians running all over the prairies slaughtering each other over religious beliefs or anything else. They certainly fought locally, but at nowhere near the level of European and Asian cultures. The first Europeans arriving in what is now laughed at the Indians because they were so frickin' clean. And the natives generously offered food and other much needed gifts to the Europeans, at least until the Europeans started shooting and stuff.
The Spanish were able to conquer the Aztecs with a tiny army, because the Aztecs had never been to war. They didn't know how to fight.
The first Europeans traveling up the Mississippi found farms along the river for hundreds of miles, and smiling, generous Indians.
Yet they had never been exposed to Christianity.
How is that possible if you say that all morals come from your religion. How could they possibly have been better human beings that Christians without being exposed to your ideals?
God wrote the law on our hearts. No surprise there. Lots of people are Christians without even realizing it. Just like how some people are non-Christians without even realizing it (the murderous ones).