Prayers Before Battle.
Let us also compare SM Stirling's Abbot with Poul Anderson's Phillipe Rochefort. For the latter's invocation of St Joan, see here. The Abbot invokes:
"Mary, pierced with sorrows..."
-A Meeting At Corvallis, Chapter Seventeen, p. 466.
Polytheists invoke gods; monotheists invoke God. However, Catholics can invoke:
the Mother of God;
three divine persons -
- thus four kinds of supernatural persons, with Mary the only such person in a class by herself.
Mary is asked to intercede. Joan is asked to help, which can only be by interceding. I accept that Rochefort and the Abbot are reminding themselves of meaningful stories but not that the persons addressed literally exist and are able to intercede. An invocation of a warrior god or saint expresses an aspiration to emulate the god or saint.