Walker Percy has a couple of interesting essays that argue Christians are the best writers because of their theology. The themes of fall, sin, struggle, redemption, incarnation, sacrament make for the finest fiction writing. They speak to the human condition in all of its complexity and tragedy. He says that this is the reason that there are no good Buddhist, Hindu, or Eastern writers as they do not understand the predicament of humanity and are incapable of speaking to it. Even those western writers who reject the faith are still formed by an understanding of history and life that moves toward something (from their denial of a Christian teleology). The existentialists and nihilists write in rejection of such a worldview, which indicates their formation by it. If Percy is correct, then poor writing among Christian traditions might be linked to weak areas of their theology (anthropology/hamartiology/soteriology/eschatology, etc.).

I would further suggest that there is a reason the best writers come from sacramental traditions.
+Mason