This past Sunday I watched the Masterpiece Mystery version of Agatha Christie's Murder on the Orient Express. It's been so long since I've read the book that I can't tell you if anything I say here also applies to it. I'm guessing the religious aspect was emphasized by this particular production.
We see Hercule Poirot's rosary twice during the show: once toward the beginning and again in the very last shot. Between the two appearances, it (or at least Poirot's Catholicism) has taken on more meaning.
I'll try not to give away the solution to the mystery to anyone who doesn't know it, but the real emphasis in this production isn't so much on the who-dun-it as it is on the self-righteous mindset that led to it. The murder victim is a person who some might say deserved what he got - not only dying, but suffering as he's dying.
After we've met the various characters, there's a scene that cuts back and forth between Poirot and the murder victim as both pray that night. The victim pleads with God for forgiveness and protection - he's already terrified that he's going to be killed. Poirot, on the other hand, thanks God for having created him and for making him Catholic. Instead of begging for forgiveness, he asks God to accept "whatever good I might have done" that day. This could come across like the parable of the tax collector and the pharisee, except for the fact that Poirot doesn't put himself above other people ("I thank you, Lord, that I am not like other men..."). I also wonder if the "good I might have done" didn't include Poirot's refusal to accept the professional job of protecting the victim, thereby forcing him into his honest pleading with God. We don't see Poirot praying the rosary, but he's holding one and kisses the crucifix at the end of his prayer.
Another important reference to Poirot's religion comes during the murder investigation, when a suspect who has "accepted Jesus" but is anti-Catholic says to him, "But the Catholics have it all wrong, don't they, with their penance and their forgiveness." Poirot asks, "Because there are some sins that cannot be forgiven?" and the suspect replies in the affirmative. (To be fair, I don't think most people who've accepted Jesus would agree - at least not the way this character means it, which has nothing to do with the sin against the Holy Spirit. I also think most would disagree with her belief that she is "without sin" so is qualified to cast the first stone.)
When we finally come face-to-face with the self-righteousness that led to the murder, it seems that it's expected that Poirot will agree that the killing was justified. He's told in an almost joking manner that his presence on the train was "the first bit of bad luck" when it came to carrying out the crime perfectly. But Poirot vehemently disagrees that taking the law into one's own hands is justifiable. I think one of the marks of a good actor is the ability to play suppressed anger and David Suchet does a masterful job; it's clear that what we see on the surface is nothing compared to the rage within.
In the last shot of the program, we see Poirot walking alongside the train. His rosary comes out of his coat pocket long enough for us to see that he's grasping it. Is it a reminder that even the sins of the murder victim could be forgiven? Or is it a statement that Poirot (still with suppressed anger) is struggling with the belief that even the self-righteousness he's witnessed is forgivable?