The Grand Master's Chapel is a Tier 2 government building in Civilization VI: Rise and Fall. It is built in the Government Plaza district and requires a Tier 2 government (Merchant Republic, Monarchy, or Theocracy) and a Tier 1 government building (Ancestral Hall, Audience Chamber, or Warlord's Throne).
The Grand Master's Chapel is a brilliant building for every civilization that has a great Faith output, as they can quickly "pray" an army into existence. Religious civilizations, who place great emphasis on spreading and defending their religion (peacefully or forcefully) should pick up this building, combined with Theocracy as the preferred Tier 2 government. These include (but are not limited to) Spain, Georgia, Mali, Ethiopia and Poland. Georgia in particular can make great use of this building, since Tamar can get Faith from unit kills. Indonesia can pick this building as well, although they do not fit the above description as much as the other civilizations. If you have good Faith output as Indonesia, you will be the only civilization in the game that can purchase both land and naval units with Faith, allowing you to quickly amass a military for domination on both land and sea. This building also has great synergy with Norway, which will be looking to pillage a lot of tiles regardless.
Although rare, religious military orders are an interesting intersection of the spiritual and the earthly. The best have been elite soldiers, and have gone on to occupy a place in the cultural imagination well beyond their historical impact. In the west, there were Crusader orders like the Knights Templar, Knights Hospitalier, and Teutonic Order. Japan produced the sohei, an order of military Buddhist monks.
In the West, the leader of a religious military order is usually called the Grand Master, and he combined the spiritual and military leadership in his person and role. A Grand Master was expected to serve as a paragon for the ideals of his followers, but was also given certain privileges and prerogatives. This may have included the privilege of a private place of worship and prayer, to which he may retire for contemplation, and to gain the strength needed to bring victory to the forces of heaven on earth.