For sacraments, the material and the signifier are necessary. It's interesting to look at good fiction in the same way: the material is the story itself, the signifier is that which sets it apart for the believer.
St. Ambrose writes of sacrament:
I see the water I used to see every day; does this water in which I have often bathed without being sanctified really have the power to sanctify me? Learn from this that water does not sanctify without the Holy Spirit...The catechumen believes in the cross of the Lord with which he too is signed, but unless he is baptised in the name of the Father, and of the Son and of the Holy Spirit he cannot receive the forgiveness of sins or the gift of spiritual grace.Bishop Flores writes of fiction:
The author allows us to hear the thoughts of the sisters as they die...I suppose a non-believer would read those thoughts and be content to say, "Well, at least they had the consolation of their faith, though whether it is true or not is another issue." A believer, reading the same thoughts of the sisters laid bare, would likely recognize in them the signifying words that point to the reality embracing the sisters. If that sounds like a description devised out of deference to the Catholic Tradition of the Sacraments, it should. For I think sacramental signification is relevant here.
Thomas Aquinas says somewhere in his treatise on the Sacraments that the matter itself of a Sacrament is not sufficiently specific to signify the purposes to which the Lord, in instituting them, wills to put them. (He is elaborating Augustine here.) Thus, the words the Lord uses in instituting the Sacraments, the words the Church has custody over, are necessary in order sufficiently to signify the intended use of the matter.
Hence, in the Sacraments, if the matter is corrupted, even the addition of the words cannot supply for it; and if the form of the specifying words is so botched that the specification is lost, then the sign fails to signify, and the sacrament is invalid... Remember also that every deliberately conceived combination of event and word has an intended audience, for who signifies meaning into the air?
In any event, a Catholic author works in the world of things and events that signify in themselves, but not sufficiently clearly for us to catch their full intended significance. So the words an author uses are at the service of signifying the intentionality of life as guided providentially by God. But without the event, there is nothing for the word to specify, and without the word, the event flounders as a vehicle of meaning.
All this is simply to say that Catholic fiction is in some sense a work that participates in the dynamic of matter and form, thing and word. And the success or failure of a work of Catholic fiction depends on how well, how fluidly and effortlessly, the combination of event and word conspire to lead the reader to ponder the meaning of the intended sign.