quality:correction, structural:expansion
The "Enforcement Across Borders" hypothesis

According to this hypothesis, the Inquisition was created to standardize the variety of laws and many jurisdictions Spain was divided into. It would be an administrative program analogous to the Santa Hermandad (the "Holy Brotherhood", a law enforcement body, answering to the crown, that prosecuted thieves and criminals across counties in a way local county authorities could not, ancestor to the Guardia Civil), an institution that would guarantee uniform prosecution of crimes against royal laws across all local jurisdictions.

The Kingdom of Castile had been prosperous and successful in Europe thanks in part to the unusual authority and control the king exerted over the nobility, which ensured political stability and kept the kingdom from being weakened by in-fighting (as was the case in England, for example). Under the Trastámara dynasty, both kings of Castile and Aragon had lost power to the great nobles, who now formed dissenting and conspiratorial factions. Taxation and varying privileges differed from county to county, and powerful noble families constantly extorted the kings to attain further concessions, particularly in Aragon.

The main goals of the reign of the Catholic Monarchs were to unite their two kingdoms and strengthen royal influence to guarantee stability. In pursuit of this, they sought to further unify the laws of their realms and reduce the power of the nobility in certain local areas. They attained this partially by raw military strength by creating a combined army between the two of them that could outmatch the army of most noble coalitions in the Peninsula. It was impossible to change the entire laws of both realms by force alone, and due to reasonable suspicion of one another the monarchs kept their kingdoms separate during their lifetimes. The only way to unify both kingdoms and ensure that Isabella, Ferdinand, and their descendants maintained the power of both kingdoms without uniting them in life was to find, or create, an executive, legislative and judicial arm directly under the Crown empowered to act in both kingdoms. This goal, the hypothesis goes, might have given birth to the Spanish Inquisition.

The religious organization to oversee this role was obvious: Catholicism was the only institution common to both kingdoms, and the only one with enough popular support that the nobility could not easily attack it. Through the Spanish Inquisition, Isabella and Ferdinand created a personal police force and personal code of law that rested above the structure of their respective realms without altering or mixing them, and could operate freely in both. As the Inquisition had the backing of both kingdoms, it would exist independent of both the nobility and local interests of either kingdom.

According to this view, the prosecution of heretics would be secondary, or simply not considered different, from the prosecution of conspirators, traitors, or groups of any kind who planned to resist royal authority. At the time, royal authority rested on divine right and on oaths of loyalty held before God, so the connection between religious deviation and political disloyalty would appear obvious. This hypothesis is supported by the disproportionately high representation of the nobility and high clergy among those investigated by the Inquisition, as well as by the many administrative and civil crimes the Inquisition oversaw. The Inquisition prosecuted the counterfeiting of royal seals and currency, ensured the effective transmission of the orders of the kings, and verified the authenticity of official documents traveling through the kingdoms, especially from one kingdom to the other. See "Non-Religious Crimes".