Like all ancient cultures, Aztec religion and Aztec mythology were closely woven into the texture of Aztec life. The Aztecs were polytheists and showed reverence to many gods and supernatural creatures in the belief that they controlled each aspect of their lives. Several myths (related with these gods and spirits) about the Aztecs – right from their origins to their arrival in their promised land and their day-to-day existence go the rounds still today.
Legends have it that when the Aztecs settled in the Anahuac valley near Lake Texcoco, they were looked down upon as an utterly uncivilized clan. With the passage of time, the Aztecs started absorbing the culture of the local tribes. They were most influenced by the Toltecs (whom they confused for the ancient Teotihuacan). They adopted most of the Toltec / Nahua gods, instilling some typically Aztec elements into them.
The Aztecs, thus, associated Huitzilopochtli (their tribal god) with the Nahuatl rain god Tlaloc. Huitzilopochtli, however, happened to be the sun and war god of the Aztec people and so the duo – Tlaloc / Huitzilopochtli were used to express the duality of water and fire.
A conquering civilization with agriculture as its backbone, the Aztec religion was based on numerous mythologies surrounding the sun and rain gods. Mythologies also refer to many rituals and rites that Aztecs practiced, to show the veneration for the earth mother goddess Tonantzin and many vegetation deities. Myths about fertility gods and goddesses, music and dance deities and natural forces also contribute to Aztec religion.
The capital city of the Aztecs, Tenochtitlan, has a famous folklore associated with it. Huitzilopochtli, according to mythology, had to kill his nephew, Cópil (whose heart he threw into the Lake Texcoco). Since Cópil was his relative, he rendered the place sacred by causing a cactus to grow over Cópil´s heart. Another myth related to the central city of Aztec civilization indicates that Huitzilpochtli had instructed his people (through divine vision) to settle in a land where they would come across an eagle devouring a snake perched atop a fruit bearing Nopal cactus.
The Aztec tradition of human sacrifice too has something to do with mythology. A popular folklore suggests that Quetzalcoatl (whom the Aztecs regarded as one of the creators of the universe) had given life to them by dripping his own blood into their bones. Rooted in this belief, Aztecs started paying their homage to Quetzalcoatl by worshipping him with human blood. Later, this became the custom and Aztecs participated in human sacrifices to appease and honor all gods and supernatural creatures.
Following this detailed account, you should have no doubt that to understand Aztec civilization you will need to have a better understanding of Aztec mythology.