At the beginning of the Middle Ages, almost all Europeans and people of Spain were Christian.
Spain's ruler was weak in power which led to the left region being vulnerable to attack.
In time, most Christians in Spain converted to Islam.
In 711, a Muslim force landed in Spain, in a few year they conquered all of Spain and Portugal. The Muslims had rewduced the Christian controlled area by a lot.
In the early 1000, the Islam rulers began to decline and there was a majority of Christian ruled land again.
During the Reconquista, 1085-1492, the Spanish developed a way of fighting on the battle field. The Reconquista was also a way for the Christian kingdoms to expand their power. The Spanish ended up having the deadliest fighting force in Europe.
King Ferdinand and Queen Isabella tried to unite Spain under one religion, Christian. The first step they took to carry out that was, they took over the Spanish Inquisition.