When I was reading and rereading Harrow, I already thought that the Mithraeum was a really cool name for God’s space station, but it’s much funnier having read Nona. The Mithraeum is a reference to the Mithraic Mysteries, an ancient Roman cult/religion about a guy who slaughtered a bull, spilled its blood and thereby renewed the world. Or something, I’m not a Classicist and the secret nature of the Mithraic mysteries mean that a lot of the sources are vague or contradictory. What we know for certain that the Mithraea were underground temples, built to look like a cave, specifically the cave where Mithras killed the bull, and that the cultists held their feasts and secret initiation rites there. So far, so good, the Mithraeum is God’s secret temple, a place where new Lyctors are initiated, and where blood is spilled, or at least where spilled blood is remembered.
But then you read Nona, and find out that John sacrificed literal cows in very large numbers, that in fact he made an inside-out cow-wall, and this reference to a mythological killer of cattle becomes pretty hilarious.