If your analysis of The Conjuring is accurate, the source of the horror (for that portion of the audience which really believes the film documents actual events) is more like that of the clip below
That is, the source of the horror is not purely theological, but empirical. The audience is (or that portion of the audience credulous enough to believe it is true) being driven to theological terror by the empirical proof of the "truths" they are witnessing. They are being "made" theological (or having their theological beliefs reinforced) by the "true events" they're witnessing.
In the case of The Exorcist, rather, they are being tortured theologically (by the baggage they brought into the theater with them). If anything, your analysis is proof of the weakening grip of theology and superstition--the audience has to be pranked into thinking it's a true story to be scared by it. In the latter case you were seeing what you believed. In the former case, you believed what you saw (because you were told it was true). The latter case more closely shows that last gasp where the audience could be terrified by beliefs they brought in with them.
That is, the source of the horror is not purely theological, but empirical. The audience is (or that portion of the audience credulous enough to believe it is true) being driven to theological terror by the empirical proof of the "truths" they are witnessing. They are being "made" theological (or having their theological beliefs reinforced) by the "true events" they're witnessing.
In the case of The Exorcist, rather, they are being tortured theologically (by the baggage they brought into the theater with them). If anything, your analysis is proof of the weakening grip of theology and superstition--the audience has to be pranked into thinking it's a true story to be scared by it. In the latter case you were seeing what you believed. In the former case, you believed what you saw (because you were told it was true). The latter case more closely shows that last gasp where the audience could be terrified by beliefs they brought in with them.