Holy Church

The hidden side of the prominent religion whose name means “universal.” In Tsukihime, it is referred to merely as “The Church.”
Those who teach the word of God do not acknowledge things which run counter to their doctrine. Though the public face of the Church simply treats these heresies as if they do not exist at all, there were zealots within the organization who felt they should be dealt with more proactively.
This enormous division that specializes in the hunting of heresy is known as the Holy Church.
They lead the devil exterminators called Executors, and exist for just two reasons: to completely expunge all heresy, and to properly manage those mysteries that are beyond human control.
Naturally, they are on bad terms with the Association, which seeks to conceal magecraft, and the two have crossed swords time and time again. (At present the two have reached a tenuous agreement, and overt hostilities have, at least for now, come to an end.)
However, their real enemies are the monsters known as “blood-suckers.” For the sake of combatting this greater threat, they have even been known to cooperate with the Association on occasion.
Kotomine is an executor attached to the Assembly of the Eighth Sacrament, the agency tasked with the management and recovery of holy relics.
The preeminently skilled warriors, the executors. The knights stationed at each individual church. And, the monsters of the Burial Agency, which headquarters does their best to keep hidden.
With these forces at their disposal, the Holy Church is the largest organization in the world of Fate/stay night, and the greatest enemy of those who can no longer be called human.

Fate/side material: Fate Encyclopedia
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The hidden side of the prominent religion whose name means “universal.” In Tsukihime, it is referred to merely as “The Church.”
Those who teach the word of God do not acknowledge things which run counter to their doctrine. Though the public face of the Church simply treats these heresies as if they do not exist at all, there were zealots within the organization who felt they should be dealt with more proactively. This enormous division that specializes in the hunting of heresy is known as the Holy Church.
They lead the devil exterminators called Executors, and exist for just two reasons: to completely expunge all heresy, and to properly manage those mysteries that are beyond human control. Naturally, they are on bad terms with the Association, which seeks to conceal magecraft, and the two have crossed swords time and time again. (At present the two have reached a tenuous agreement, and overt hostilities have, at least for now, come to an end.) However, their real enemies are the monsters known as “bloodsuckers.” For the sake of combatting this greater threat, they have even been known to cooperate with the Association on occasion.
Also, there are various jurisdictions within the Church as well. The Burial Agency, which Ciel belongs to, are like the elite commandos of heresy hunting.
Arcueid, Roa, and Ciel all have deep ties to the Church, but since Tsukihime is set in the Far East outside of the Church's influence, it serves as little more than a backdrop to the story's events.

Tsukihime Dokuhon Plus Period: Tsukihime Dictionary Revised
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The hidden side of the prominent religion whose name means “universal.” In Tsukihime, it is referred to merely as “The Church.”
Those who teach the word of God do not acknowledge things which run counter to their doctrine. Though the public face of the Church simply treats these heresies as if they do not exist at all, there were zealots within the organization who felt they should be dealt with more proactively. This enormous division that specializes in the hunting of heresy is known as the Holy Church.
They lead the devil exterminators called Executors, and exist for just two reasons: to completely expunge all heresy, and to properly manage those mysteries that are beyond human control.
Naturally, they are on bad terms with the Association, which seeks to conceal magecraft, and the two have crossed swords time and time again. However, their real enemies are the monsters known as “bloodsuckers.” For the sake of combatting this greater threat, they have even been known to cooperate with the Association on occasion.
Also, there are various jurisdictions within the Church as well. The Burial Agency, which Ciel belongs to, are like the elite commandos of heresy hunting.

MB Act Cadenza PS2 Manual: Tsukihime Dictionary
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The organization that ran contrary to the “universal tolerance” of the religion.
Preaching the word of God, they absolutely did not tolerate anything against the doctrine. Although on the surface they denied the existence of “heretics,” within the ranks there existed some who rejected heretics with unmatched fervor. They turned “heretic hunting” into a profession, and a immense department was established for this purpose.
“Executors” were individuals with the power to vanquish Demons. They physically eliminated things that did not exist in the Lord's teachings. On the other hand, Exorcists were individuals in the clergy who tolerated the existence of Demons, but merely temporarily repelled them.

Character Material: Prelude Explanation