Because of the Aztec's religious beliefs, when Cortes arrived in Mexico, the Aztecs believed that he was a god. The horses that the men rode and their much lighter skin coincided with a religious belief that they had and so, instead of trying to fight off Cortes and the Spaniards upon their arrival, the Aztecs treated them as gods; lavishing them with gifts and giving them food, gold and other supplies.
Cortes had an advantage because there was a god by the name of Quetzalcoatl who resembled a Spaniard, in other words he was fair in complexion and had blue eyes. Now the Aztecs believed that Quetzalcoatl would one day return to Earth. So when Cortes came to conquer them, they thought he was Quetzalcoatl returning to Earth. So Cortes could basically make them do whatever he wanted, by bringing up the fact that he was their god.
Impact of Aztec religious beliefs