This was a time of ongoing religious wars known as "Holy Wars". The capital of the Byzantine Empire, known as Constantinople, had fallen at the hands of the Turkish Ottoman Empire, who practiced the Muslim faith.
As a response, the western Christians gathered in arms and started a series of military campaigns known as "The Crusades" with the objective of retaking territories that were currently under Turkish (Muslim) control in the name of God. Spain had been one of these conquested territories, so at the time, people living in the Iberian Peninsula could be either Christian or Muslim.