WARNING: A bit gory, though no reconstructions are shown.
Earlier I mentioned that it was possible for the victims of Aztec heart-sacrifice to maintain consciousness during the process, and that the heart, even after it was ripped from the chest, could still be beating. I checked up on this and found a relevant video from the History Channel. It confirms both statements.
These sacrifices, of course, were made in the name of religion. And they were numerous: some estimates are as high as 5,000 victims killed in a single day when a temple was consecrated, and many more thousands as quotidian sacrifices during the year. (For two other, longer videos on this topic, with reconstructions of the practice by surgeons, go here and here.)
One thing that struck me visiting the pre-Columbian temples around Mexico City is the number of gods they worshiped and which had to be propitiated with blood. Yet, as H. L. Mencken pointed out in a famous essay, those gods are dead: almost nobody believes in them any more. They are ex-gods, bereft of life.
It would be interesting to ask a theologian why he or she absolutely denies the divinity of Huitzilopochtli or Quetzalcoatl. If said theologian were a Christian, Jew, or Muslim, I suppose they’d cite the Qur’an or Bible. But that’s only because there was no formal written Aztec scripture—only engravings, murals, and word of mouth.