The Inquisitor is a religious unit in Civilization VI. It can only be purchased with 
Faith in a city with a majority Religion after an Apostle has launched an Inquisition, and requires a Holy Site with a Temple.
Attributes:
- Action: Remove Heresy (3 charges)
- Ability: Engage in Theological Combat
Strategy
Inquisitors are the Defenders of the Faith, specializing in removing competing religions from your cities. They start with 3 charges of Remove Heresy (4 as Philip II). Using one charge in a City Center tile removes all religions from that city, besides your own. You cannot use the ability from a tile adjacent to the city, unlike spreading religion.
The mechanics of Religion permit now that a particular Religion is practically eliminated from the game by enemy Religious agents converting away all of its cities with a Holy Site. This makes it impossible for Religious units of this religion to be produced, and from that moment on, it's only a matter of time until this religion is completely wiped out. That's why it is always a good idea, when facing strong religious opponents, to have an Inquisitor or two stashed away, ready to convert back at least the Holy City, so that you can start producing religious units again.
Inquisitors can also initiate Theological Combat. Inquisitors receive +50% 
Religious Strength if battling in friendly territory, which makes them ideal for withstanding massive attack waves of enemy Apostles. Note, however, that the Inquisitor's Religious Strength is the lowest of all religious units, so avoid using it for combat outside your territory.
Historical Context
Every faith has its inquisitors, those charged by the church to find the non-believers, back-sliders, and evil influences among the flock. But it is in Europe that the role reached its apex, as the Catholic Church during the Medieval and the Renaissance periods strove to insure the Jews, Moslems, and Protestants (not to mention scientists, artists, and philosophers) didn’t corrupt the faithful. The Catholic inquisitors were on the front lines in this struggle, as they banned and burned books, and disposed of heretics, all in the name of the Lord. In some cases, with the support of the secular government, as in Isabella’s Spain or the Holy Roman Empire, all made legal by the papal bull 'Ad extirpanda' in 1252 AD. Despite some excesses it can be said that inquisitors did keep the faith – whichever faith it may be.