The Spanish Inquisition was an institution that had precedents in other Inquisitions. The reconquest of Spain from the Moors resulted in a relatively peaceful multi-religious society, but violent anti-Judaism ensued and Jews converted en masse to the Catholic faith. Some of these conversos were suspected of not being sincere converts and the Alhambra Decree in 1492 ordered all Jews to leave their kingdoms, making an unknown number leave. Various motives have been proposed for the monarch to start the Inquisition, such as increased political authority, weakening opposition, doing away with conversos and sheer profit.
Ferdinand II of Aragon pressured pope Sixtus IV to agree to letting him set up an Inquisition controlled by the monarchy by threatening to withdraw military support at a time when the Turks were a threat to Rome. Sixtus IV later accused the Spanish inquisition of being overzealous, accused the monarchs for being greedy and issued a bull to stop it, but he was pressured into withdrawing the bull. On both occasions Sixtus IV went along with Ferdinand II of Aragon.
During the 16th century the main target became Protestants and about 100 were executed. An index of prohibited books was drawn up that were alleged to contain heresy. In time converts from Islam and Moriscos were also persecuted by the Spanish Inquisition. Other crimes were also taken up in these tribunals such as Superstitions, heretical propositions, Sodomy, Bigamy and solicitation. Among superstitions, six witches can be positively verified to have been executed.
The Spanish Inquisition was an institution at the service of the monarchy, but had to follow procedures set up by the Holy See, and the appointment of Inquisitor General had to be approved by the pope. Most of the inquisitors had a university education in law. Its income came from the confiscations and otherwise operated in conformity with Canon Law of the Roman Catholic Church. The procedures would start with Edicts of Grace, where people were invited to step forward to confess heresy freely and to denounce others. Denunciations were followed by detentions, which in some cases lasted up to two years before the trial. A defense counsel was assigned to the defendant, a member of the tribunal itself, whose role was simply to advise the defendant and to encourage him or her to speak the truth. A Notary of the Secreto meticulously wrote down the words of the accused. The archives of the Inquisition, in comparison to those of other judicial systems of the era, are striking in the completeness of their documentation. The percentage of cases where torture was used, which was as a means of getting confessions, varied. Sentences varied from fines to execution and those condemned had to participate in the ceremony of auto de fe. The arrival of the Enlightenment in Spain in the first half of the 18th century slowed inquisitorial activity and it was definitively abolished on July 15, 1834. From 1476 to 1834 probably between 3,000 and 5,000 people were executed.
In the mid-16th century to the mid-17th century, a time when Europe was torn apart by Catholic-Protestant strife, there began to appear from the pens of various European Protestant intellectuals, who generally had minimal or no direct access or experience of the Inquisition, what has come to be known as the Black Legend, as part of the Protestant polemic in support of the Reformation. With the gradual ebbing of religious hostilities professional historians began investigations, giving a detailed, nuanced and less exaggerated picture of the Inquisition.