In August 1519, Hernan Cortes, a Spanish Conquistador, arrived in Tenochtitlan, the mighty Aztec capital. The Aztecs didn't understand Cortes and his men because they spoke Spanish while the Aztecs spoke Nanuatl.
The Aztecs welcomed Hernan Cortes and thought that he was a bearded Aztec god that brought peace. The Aztecs learned that Cortes was a Catholic which is a form of Christianity, and Hernan Cortes learned that the Aztecs were polytheistic.
To get more power and gold Hernan Cortes kidnapped and killed the emperor, Montezuma.