The Sentinel were originally founded by one of the Draic Kin: given the Red's ongoing involvement with them, he was most likely responsible. The Sentinel were instrumental in dividing Earth into two "worlds": Stark and Arcadia; science and magic. They set up a Guardian in charge of controlling and channeling the Balance of science and magic between the two worlds; a Guardian of the Balance who lives in a tower in the Guardian's Realm, and who is replaced every one thousand years by a new Guardian. The founding Kin gave them a Stone Disk that essentially functioned as a 'spare key' to the Guardian's Realm, to be used in the event of an emergency.
After many generations of Guardians, most of the Sentinel priests of Stark broke away from their brethren in Arcadia to form the Vanguard. For thousands of years the Stone Disk was kept on display at the Sentinel Enclave in Marcuria, until Tyren agents of the Vanguard attempted to steal it. They were thwarted by the White Dragon, even though she was forbidden from interfering in such matters.
At the time of the events in TLJ the Sentinel are few in Stark, represented in the game only by Father Raul. They are more of a power in Arcadia, but no-one knows what their position is after the Tyren invasion of Marcuria.
The Sentinel have a temple in the central square of Marcuria, and the Enclave on the outside of the town, containing one of the greatest libraries in the world.
Their canonical books are known as The Scriptures of the Balance, which number 13. Of these, the 2nd is known as 'The Scripture of Song' and the 8th is known as 'The Scripture of Breaking' (of which the 16th chapter 'Regarding the Disc of the Balance and the events that came about when the disc was broken' is readable in TLJ).
There appear to be 3 ranks of the Sentinel hierarchy:
In Dreamfall the Sentinel are being persecuted by the Azadi,and their temple has been demolished. April calls it as the Temple of the Balance, and refers to it as their "holiest shrine". The term "Fathers" also appears to have been dropped altogether, however, and suddenly we are presented with the problem of the very female Minstrum Magda in their ranks.