The drunken Irish priest experiencing a crisis of faith is such a tired cliché that you almost expect the entire rest of the cast—Barlow, Ben, Matt, Susan, Mark—to just laugh in Callahan's face as soon as he shows up. "Really? You're the best the author could give us? Come on, man; try again."
Barlow does eventually laugh in Callahan's face: "The boy makes ten of you, false priest" (14.518). It's true: Mark's maybe not the greatest character in all of literature, but he sure looks good compared to this walking stereotype. Being defiled, humiliated and forced to suck Barlow's blood has to be the best thing that could possibly have happened to Callahan—a bizarre, perverse finale for this kind of boring due.
Unfortunately, Callahan pretty much just goes whining and moaning and drinking his way out of town. But at least he is headed out of town and out of the book. He could have stayed and helped with the good war even though he lost a battle…
Except he's still not actually gone. Of all the characters whose story King could have continued, it's Father Callahan who gets sequels. He shows up in a bunch of King's Dark Tower novels—The Dark Tower V: Wolves of the Calla, The Dark Tower VI: Song of Susannah, and The Dark Tower. There his struggle with his faith and self-doubt goes on.