The Aztecs had a very complicated system of religious beliefs, involving many gods (polytheistic), as opposed to the most European religions, which believed that there is only one god (monotheistic). The basic beginning of the Aztec creation myth is that Coatlique (Lady of the Serpent Skirt) gave birth to a daughter (the moon) and sons (the stars). Later, she gave birth to Huitzilopochtli, the god of war and the sun. Huitzilopochtli then killed the gods of the moon and the stars, because the latter were planning to destroy Coatlique. The Aztec creation myth then goes on to include many other gods and goddesses. Among them are: Centeotl, the god of corn; Ehecatl, the god of wind; Mictlantecuhtle, the god of the dead; Quetzalcoatl, the god of learning and civilization; Tlaloc, the god of rain; Tlaloc's sister Chalchihuitlicue (a statue of whom is in the picture on the right); and Xiuhtecuhtle, the fire god.
These gods, and many more, were praised and revered by the Aztecs as part of their everyday activities. Prayer was an integral part of the Aztec culture, and it was not uncommon to pray for anything from health to success at war. Another common prayer was one for life. As odd as that sounds, it was quite logical to the Aztecs. Revering their gods above all, the Aztecs considered themselves tiny and wretched beings. Therefore, they often prayed to the gods to let them live. Since some theories hold that the Aztec population was relatively healthy (before the coming of the Europeans), it is likely that the Aztecs took this general health as a sign that their prayers were received. They then continued to pray, with even greater zeal. The most common gods that were prayed to were Quetzalcoatl, the god of civilization and learning, and Huitzilopochtli, the god of war. The gods of the sun, rain, and corn were also frequently offered prayer, as well as the less important gods. Besides prayer, the ways to honor the gods were by offering animal sacrifices, as well as pricking oneself so as to bleed.
Legend has it that Aztec priests often performed human sacrifices. In these sacrifices, the usual victims were prisoners of war. In more important ceremonies, however, Aztecs of high rank, especially warriors, volunteered themselves to the gods. During the course of a year, the death toll allegedly rose to thousands. There is, however, an opposing viewpoint. The only written accounts of these sacrifices are by either Hernan Cortez or by his historian, Berna Diaz del Castillo. Taking into account the absolute lack of any moral code whatsoever on the part of Cortez, it seems very possible that he lied about the Aztecs in order to justify his murderous conduct with them. It is also possible that what Cortez witnessed was not human sacrifice, but was rather the Aztec form of the death penalty. Since Cortez did not see more than several of these "sacrifices", the latter possibility is quite likely. At the time, Cortez' knowledge and understanding of the Aztec culture was not nearly enough to tell whether those sacrificed were prisoners of war that were being sacrificed to the gods, or whether they were men who were sentenced to death due to some crimes that they had committed, such as murder. Then, there is always the possibility that Cortez lied about what he saw, and that no such executions/sacrifices ever took place. One possible scenario is that Cortez needed to find some way to justify his behavior in dealing with the Aztecs, which was appalling at best. There is no better way, it seems, to justify murderous behavior than to say "He did it too!" That is, in effect, what Cortez could have done. The only written accounts about these "sacrifices" are by Cortez and his historian del Castillo, and the possibility that these two either lied about what they saw (or didn't see) or were misled by seeing simply executions, that probability is quite high.
The Aztecs followed a religious calendar of 260 days, which combined with their scientific calendar of 365 days to make a 52 year cycle. At the end of each cycle, the Aztecs would put out their hearth fires and light new ones, to signify the new cycle. Naturally, feasting and celebrations followed.
The Aztec religion was closely intertwined with their sciences, such as astronomy and medicine. Agriculture and farming were also related to religion: since Aztec society was based on farming, the most important gods were the gods of rain, corn, and wind.