The Chapter of Saint Anthony was a religious organisation.
In the mid-21st century, all of Earth's religions merged and became the High Catholick Church in an attempt to create world peace. Instead, greater dissent was the result, so the Church was dissolved. The Chapter filled the void that resulted.
Soon after, they departed the Earth, building a battlecruiser that looked like a cathedral. Led by the Magna, they travelled from world to world on a crusade to convert the inhabitants to their religion. Those that refused were incinerated. Some of the natives were harvested and brainwashed to become new members of the Chapter. Civilizations that they destroyed included Urrozdinee and Massatoris.
When they tried to convert Betrushia, they destroyed the rings surrounding the planet. This caused the Keth to be unleashed from its confinement. The Seventh Doctor convinced the Magna to help him destroy Betrushia and the Keth so it would not endanger other planets.
Unknown to the Doctor, Bernice Summerfield convinced one of the Chaptermen to disable the mothership's navigation. Unable to break orbit, it was vaporised along with the planet. (PROSE: St. Anthony's Fire)