This is one of the best mysteries I've seen in a while, perhaps because it reaches beyond being a simple whodunit and becomes a complex, personal drama.
One of the most intriguing aspects of Death in Holy Orders is that you know the characters better than they know themselves, and certainly better than they know each other. The knowledge we have of them propels the story forward easily as you analyze their criticisms of each other and weigh their motives. The dialogue borders on poetry at times, and Martin Shaw in particular executes the lines with compassion and honesty.
My only two criticisms are a subplot that made me rather queasy, and the actress who played Inspector Kate Miskin (very, very smugly). If an officer walked around looking at criminals that way in real life, he or she wouldn't make it to Inspector.
In addition to Martin Shaw's natural execution, the performances of Robert Hardy, Clive Wood, and Jesse Spencer (where did this kid come from?!) are a real treat in this film.