The writer who undertakes the description of any of the great faiths of the world usually presupposes in his readers a certain acquaintance with the history and conditions of the people of whose religion he treats. But the obscurity which surrounded all questions relating to Mexican antiquity until the beginning of this century formerly made it essential that any view of its religious phase should be prefaced by an account of the peoples who professed it, their racial affinities, and the country they occupied. This necessity no longer exists. The ground has been traversed so often of late, and I have covered it so frequently in previous works, [3] that I feel only a brief account of these conditions is necessary here, such, in a word, as will enable the reader to realize circumstances of race, locality, and period. The people whose religious ideas this book attempts to describe were the Nahua of pre-Colombian Mexico, a race by no means extinct, despite the oft-repeated assertions of popular novelists, and which is now usually classed as a branch of the great Uto-Aztecan family of the North American Indian stock. They spoke, and their descendants still speak, a language known as the Nahuatl, or Nahuatlatolli (“speech of those who live by rule” or “by ritual observance”). At the era of the Spanish invasion of their country in 1519 they had succeeded in overrunning and reducing to their dominion practically all that part of modern Mexico which lies between the Tropic of Cancer and the Isthmus of Tehuantepec. They were, in all probability, immigrants from the north, and their art-forms, no less than their physique and beliefs, have led certain writers to form the opinion that they came originally from the neighbourhood of British Columbia, or that they had a common origin with the Indian tribes which inhabit that region at the present time.