The protagonist in our current read, Eagle in the Snow by Wallace Breem, Paulinus Gaius Maximus, is still a Pagan in a by now largely Christian world. Like many soldiers in an earlier Rome he worships the God Mithra.
Our friend Bingley tells us all we wanted to know and more: Mithraism.
To add to this, Britannica online has this to say: Mithraism.
The image on the right from Wikimedia Commons represents Mithras as the Sun god banqueting with Selene (the Moon) and the twin divinities Cautes (Dawn) and Cautopates (Twilight), his attendants. Marble, side B of a two-faced Roman relief, 2nd or 3rd century AD. (The Louvre). Bingley's post shows side A, Mithras killing a sacred bull (tauroctony).