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}
Virtual Mithraeum at the Museum of Antiquities, University of Newcastle-on-Tyne, England
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
A Hymn to Mithras by Rudyard Kipling
Overview of Mithraism
Representations of Mithra:
Fresco of Mithra
Carving of Mithra
Photo of Mithra © Marie-Lan Nguyen / Wikimedia Commons