>>Pondering over the obvious paradox
Akin to the famous Cretan paradox of Epimenides. But a story has to build from the starting point of the audience's common perception or viewpoint. In this case, the conventional roles of God and Satan, good and evil in Catholic theology.
While we're wading into theology this logline reminds me of the story of Job where the devil gets personally involved in the life of Job. But he gets involved to win a wager over an important spiritual issue: that Job obeys God only because it's a good deal for him to do so-- he's richly blessed. Take away all the he has, Satan argues, and he will curse you. So God allows Satan to take all he has including his family.
What's your theme? What spiritual issue are you trying to explore in your story? What itch are you trying to scratch in pitting the devil against the priest like this?