I’ve been going through Cumont’s Textes et Monumentes and adding material from it to my page of literary sources for Mithras. One rather interesting snippet appeared only in a footnote, and was a quotation from Ambrose of Milan’s letters (which, fortunately, I have online), addressed to the emperor Valentinian.
It seems that Ambrose knew so little about the cult of Mithras, in the late 4th century, that he supposed the deity to be female, and a synonym for Venus! Cumont comments (correctly, I suspect) that Ambrose is simply borrowing a line from Herodotus. But it does tell us that the cult had vanished from public view at this date.