The Church

The Church was a faith-based military organisation primarily operating in the 51st century. It was responsible for interstellar security, protecting human populations, and special covert operations. Instead of regular military ranks, the Church operated under more religious offices such as Bishop, Cleric and Verger.

History
The Church battled against Lava snakes. (DW: The Time of Angels)

Father Octavian recruited River Song for a covert mission in which she infiltrated the Galaxy class Starliner, Byzantium as it contained a Weeping Angel. She was able to gain the Doctor's attention which lead to him rescuing her and accompanying her to Alfava Metraxis, where the Byzantium had crashed. There they united with Octavian. Octavian led his men into the Maze of the Dead below the Byzantium, Christian, Angelo and Bob were soon killed by the Angel. The Doctor assured him they could not have been saved, but Octavian reminded him that he would be the one to tell their families while the Doctor went off in his blue box. Nonetheless he still told the Doctor he had faith in him when they were surrounded by Angels. (DW: The Time of Angels) A crack in space-time was discovered in the forest of the Byzantium. Although the Doctor, Amy and River were able to survive and send the Weeping Angels falling into the crack. Octavian was killed by a Weeping Angel and the remainder of the Clerics were erased by the crack. (DW: Flesh and Stone)

Since the Doctor negated the cracks, (DW: The Big Bang) the fates of the Clerics and the circumstances of the Angels defeat is unclear.