Paulinus Gaius Maximus, the narrator of Wallace Breem's "Eagle in the Snow" is a follower of Mithras, a god popular with Roman soldiers. Although a connection with Mithra, a god in the Persian pantheon, seems obvious at first sight and was the general scholarly consensus at the time Wallace Breem was writing, it now appears that although the name may have been taken from the Persian god, the religion as far as we can re-construct it seems to have been a Roman development with few connections to the Persian religion. (wikicommons public domain picture by Marie-Lan Nguyen, from the Nersae Mithraeum)
An overview of what we know about Mithraism can be found in
an essay by Alison Griffith. One of the key figures in the modern re-assessment of our knowledge of Mithraism is David Ulansey, who based on Mithraic iconography, ties Mithraism closely to astrological beliefs of the time.
Iconography is important because Mithraism was a mystery religion whose devotees swore not to reveal details to outsiders. Hence our written sources stem from hostile outsiders, some but not all contemporary Christians.
LacusCurtius points us to Ceisiwr Serith's pages on Mithraism, which give another overview of what we know, and just as importantly debunk some of the misinformation about Mithraism floating about, particularly with regard to the relationship between Mithraism and Christianity. Since Ceisiwr Serith is a Wiccan, he cannot be accused of a Christian bias in assessing the evidence.