The downfall of the Aztec civilization was the result of many factors, including their religion. Other factors included disease and the lack of loyalty among the people.
When the Spaniard Hernan Cortes arrived in Mexico in 1519, he was treated as an honored guest as opposed to an enemy. The Aztecs' customs called for their guest to be treated special, but it was Cortes resemblance to Quetzalcoatl that caused them to regard Cortes and his crew as an honored guest. Quetzalcoatl was a deity who the Aztecs religion taught was coming back in a human form. Cortes was able to befriend a number of people who joined him in fighting against the Aztec rulers.
Smallpox killed a number of Aztec people. The disease was possibly contracted from one of the Spaniards that traveled with Cortes. Once the fighting broke out between the Spaniards and the Aztec people, there were fewer people left to defend the empire. The Aztecs also had less superior weapons than the Spaniards. As a result, Cortes was able to quickly overtake the Aztecs and gain control of key cities, such as Tenochtitla. The Aztecs were able to temporarily regain control of Tenochtitlan, but the Spaniards fought back and conquered the city. An estimated 240,000 people were killed in the fight for the Aztec empire.