In Cortes' second letter to the Emperor he discusses his battles with the Aztecs within the city limits of the Aztec capital, Tenochtitlan. While writing about the inner-city war between the massive army of Aztecs and the handful of Spaniards, Cortes' credits his victories to the bravery of himself and his troops. Not to mention, throughout his letter of conquest, Hernan Cortes brings up God. This in turn, gives me the impression that he, like most Christians during this time, also credits God for the victory. Simply saying that they were enforcing God's will.
I on the other hand, mainly credit Cortes' destruction of the Aztec empire to the Spaniards' advanced weapons, military intelligence, and a hint of luck. While the Spanish were using guns to slowly mow down the constant waves of attacks of the Aztecs, the Aztecs themselves were armed with mere rocks. This creates a mild advantage for the gun bearing Cortes and his men, but not too large of one, due to the limited number of Spanish men and the almost infinite number of Aztecs. This is where I believe a lot of luck came into play for Cortes, the fact that he and his men fought their way out of the island city of Tenochtitlan is simply amazing. To say that you are carrying out the will of God is quite hasty and cannot be supported either way. This in turn means that the "something else" that aided Cortes in his victory can only be explained as luck at the moment.
Since there is no real way to discover the causes of Cortes' upset victory of the Aztec empire, one must figure it was the weaponry, intelligence, and luck. No matter what it was, it definitely was pivotal in the conquest of Mexico for the Spanish empire.