In hindsight, how effective was the Spanish Inquisition?

First things first, and I always have to remember this, odd as it may sound: the Inquisition was, above everything, a Court of Law. And by that I mean that the courts were formed of judges (people with a licenciate degree in Law, but preferably Doctors of Law, even better utraque iure), subject to rules, and procedural guarantees like the accused having the right to an attorney and if you could not afford one, the Court would appoint one for you, the right to repeal judges, attorneys, accusers, or witnesses, on the grounds of manifest enemity or hostility, etc.

What was the main goal of the Inquisition? Religious uniformity, the whole country being of one religion only, namely Roman Catholicism. This was especially hard after the Protestant Reformation. It is also important to note that there were no legal troubles for muslims or jews openly being muslims or jews. Having converted to Catholicism but secretly practising Islam or Judaism was severely punished, it could end in an auto de fe, with you executed and a sambenito (penitent's robe) being hung in the major church of your village for decades, shaming your family with all the trouble it implies. Martín de Enzinas, for example, was sambenitado in the auto de fe of 1496, and executed for secretly practising Islam. It was not until the 1550s that his descendants managed to have the sambenito taken down. Bear in mind that the Enzinas lineage were rich merchants, related to the most prominent families in Burgos (like the Santa Cruz, Salamanca, Cartagena, Lerma) and Juan de Enzinas and his brother Álvaro were bankers for Charles V. For less powerful families, it was more difficult, with the sambenitos staying just about forever.

Let us not forget that the conversos or descendants of conversos could not hold public office until the 4th generation. This stemmed from the infamous "Statutes of blood cleanliness" set by the Archbishop of Toledo and Inquisitor General Cardinal Martínez Guijarro. In Burgos, however, they were never enforced, as nearly all of the public offices were held by descendants of conversos. So much so, that the Constsble of Castile straight told the Inquisitor that if he wanted to enforce them, ge should come at the head of an army, for he wouldn't do it as enforcing the Statutes woyld leave the city without aldermen, councilmen, justices, or consuls of commerce. Was the Inquisition succesful in catching conversos? Relatively yes, but with the Head of Castile being the exception, it is all so very relative. The number of false converts was always very low, so in that regard the Inquisition did not have that much to do.

Fighting protestantism was also a big thing, as it was also a threat to religious unity. The nunber of protestants in the first half of the XVI century was low, as protestantism was an upper class phenomenon related to the intelectuals: the most important protestant circles in Spain were in Seville, Valladolid, and Alcalá de Henares. In Seville, it was lead by Constantino Ponce de la Fuente, who had worked for the Inquisition censoring books, and was an emminent theologian. In Valladolid, the leader was Agustín Cazalla, who had been a preacher for the Emperor no less. In Alcalá, the reference was the renowned printer Miguel de Eguía and his family. Alcalá's circle died out, but the circles of Valladolid and Seville lived until the Inquisition got to them. The autos de fe of Seville and Valladolid were terrible, half a hundred people were executed for heresy, and they were very influential people. In Valladolid, the Vivero family was nearly wiped off the face of the Earth, and they were very influential people, with a nice palace where the Catholic Monarchs had been wed in 1469. In Seville, many rich people were affected too. These two autos de fe were such a demonstration of power, that they just about eradicated protestantism from Spain. So, in that regard, the Inquisition was extremely succesful.

As time passed, the Inquisition became a powerful tool for the Crown, because the Inquisition had jurisdiction in both the Crown of Castile and the Crown of Aragon, leading to the Inquisition becoming a sort of federal institution. With this mind, it is hardly a surprise that the Inquistion was given the authority not only on religious matters, but also on fighting coin forgery, piracy, and eventually even high treason.

All in all, we can conclude that the Inquisition was very succesful in their business, as protestantism became a non-existent thing in Spain, and eligious uniformity was succesfully implemented.

Sources:

Kamen, Henry (1999), "La Inquisición española, una revisión histórica." Barcelona: Crítica.

Menéndez y Pelayo, Marcelino (1882), "Historia de los heterodoxos españoles". San José: El siglo futuro.

Amézaga, Elías (1966), "Auto de fe en Valladolid". Bilbao: Gráficas Ellacuría.

Bergua Cavero, Jorge (2005), "Francisco de Enzinas: un humanista en la Europa de Carlos V". Madrid: Trotta.

Thomas, Werner (2001), "Los protestantes y la Inquisición en España". Leuven: University Press.