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An inquisitor was an official (usually with judicial or investigative functions) in an inquisition – an organization or program intended to eliminate heresy and other things contrary to the doctrine or teachings of the Catholic faith. Literally, an inquisitor is one who "searches out" or "inquires" (Latin inquirere < quaerere, 'to seek'). Inquisitors sought out the social networks that people used to spread heresy. There were accounts where the Inquisition could not tell who was a heretic or devout, and they were killed anyway. One of these accounts was Arnaud Amalric at the storming of Béziers. The abbot was recorded as saying “Kill them. For God knows who are his.” This brought up concern about the role the Inquisition was playing and whether or not it was a truly righteous cause.
The role of the Inquisitor was further brought into question by a work of satire by author Dostoyevsky. In this prose poem, Christ came back to earth and was imprisoned by the Grand Inquisitor. The Grand Inquisitor argued that Christ couldn't be free because his work would directly oppose the church, because free will was a burden to humanity. Dostoyevsky ends the poem by saying that the Roman Empire secretly followed the work of Satan instead of Christ, due to Satan allowing the best form of order for Humankind.  In this story, the Roman Empire used Religion as a way to control the average population. This made the Inquisition out to be a war on ideology and free will, as opposed to a suppression of heresy.
Some of the better-known inquisitors throughout history include:
- Peter of Verona (also known as Saint Peter Martyr), whose canonization was the fastest in history
- Pedro de Arbués
- Nicolau Aymerich author of Directorium Inquisitorum
- Stephen of Bourbon
- Arnaut Catalan
- Fabio Chigi (later Pope Alexander VII)
- Diego Deza
- Bernard Gui
- Heinrich Institoris, author of Malleus Maleficarum
- Francisco Jimenez de Cisneros
- Konrad von Marburg
- Sebastien Michaelis
- Giovanni Pietro Carafa (later Pope Paul IV)
- Jacob Sprenger, purported co-author of Malleus Maleficarum
- Tomás de Torquemada
- Martín García Ceniceros
- Grand Inquisitor
- Medieval Inquisition
- Spanish Inquisition
- Portuguese Inquisition
- Roman Inquisition
- Mexican Inquisition
- Inquisitorial system, a type of legal system
- Ormerod, Paul; Roach, Andrew P (2004-08-15). "The Medieval inquisition: scale-free networks and the suppression of heresy". Physica A: Statistical Mechanics and its Applications. 339 (3): 648. arXiv:cond-mat/0306031. doi:10.1016/j.physa.2004.03.020. ISSN 0378-4371.
- "The Project Gutenberg E-text of The Grand Inquisitor, by Feodor Dostoevsky". www.gutenberg.org. Retrieved 2020-11-17.
- Riemer, Neal (1957-07-01). "Some Reflections on the Grand Inquisitor and Modern Democratic Theory". Ethics. 67 (4): 249–256. doi:10.1086/291124. ISSN 0014-1704.