The Catheric faith was an ancient religion of Old Earth still practiced in some form during the early 31st Millennium at the time of the Horus Heresy. The Catheric faith was the descendant of the Roman Catholic Church, and those who still practiced the religion during the early days of the Imperium of Man were looked down upon by others because of their refusal to accept the atheistic doctrines of the Imperial Truth.
A known pious Catheric of this period was Ollanius Persson, a former Imperial Army trooper and Perpetual who had retired from his service in the Great Crusade to the world of Calth in the Realm of Ultramar in the hope that being on the Imperial frontier would make it easier to practice his ancient religion.
It is unknown if the Catheric faith survived in some form after the founding of the Imperial Creed as the state church of the Imperium in the 32nd Millennium.
- Know No Fear (Novel) by Dan Abnett, pp. 32-34