Our protagonist John and his friends worship Mithra, a dangerous practice in an officially Christian court and one meaning they must worship in secret.
The links below provide background information on their religion as well as providing an overview of scholarly discussion of Mithraism in general.
The Mysteries of Mithra by Franz Cumont (1903}
Essay on Mithraism by Alison B. Griffith, Ecole Initiative
Mithras in the Encyclopaedia Britannica (1911)
Mithraism in the Catholic Encyclopaedia (1911)
German website on the cult of Mithra
Father Bianchi's Tale by R. H. Benson deals with Mithraic presence in church -- includes a brief mention of beliefs. Can be read online in collection titled A Mirror of Shalott by Robert Hugh Benson
Overview of Mithraism
Feature on Mithraism in Archaeology Magazine
Mithran Sanctuary Discovered in France
Essay on a Mithraic ritual by G. R. S. Mead
Mythraic Myths: Mary writes about what ancient misrepresentations about the Mithraic religion can teach us about religious misapprehensions today.
Representations of Mithra:
Carving of Mithra
Photo of Mithra © Marie-Lan Nguyen / Wikimedia Commons