The novel is easily superior and the best parts of the film are direct translations from what Greene wrote; for instance the quiet but grim humour that breaks into the scenes with Boyer and Lorre, or the murdered-child obsession that takes over some of the plot. Where the film deviates from the novel, it tends to the ludicrous.
However I don't want to suggest that the film is bad in any way. It always looks the part and the story stays in the mind like a good 'un. Some of the minor characters were stock actors who could turn their hand to anything.
It's a dreadful shame that the film's not available on DVD.

Honestly, this is easily in the top 5 of the worst movies I have ever seen. Partly, because it takes itself so seriously, as opposed to regular light hearted trash, this movies wants you to be emotionally involved, to feel for the characters, and to care about the alleged conspiracy. None of this ever even comes close to happening.
****MILD SPOILERS******
There are 3 main reasons why this movie is so terrible: 1.) Incoherent and totally non-sensical plot. 2.) Annoying style-over-substance "MTV" camerawork. 3.) Moronic characters and plot holes.
Allow me to elaborate.
1.) Apparently, when this movies was being made, they couldn't decide whether to make a movie about church conspiracies, the stigmata, or possession. So, guess what? They combined them! An aetheist gets possessed by a dead person, who then makes her exhibit the stigmata so as to expose a church conspiracy. How a regular person is able to transcend death and possess another human being through his rosary is never explained, nor even talked about. Now, instead of just saying what he wants to say, he gives her the Stigmata. WHY? Why not just spit it out? Instead, we get treated to scenes of screaming things in harsh voices, carving cryptic messages on cars, and writing messages on walls. Apparently this priest was also a violent guy, because the possessed young lady also wigs out on one o f the characters, while talking in that cliched, harsh, "possessed" voice that we all have heard countless times. This also starts to tie into my second complaint, because whenever the young lady gets the stigmata, she also defies the laws of gravity by floating into the air, and tossing everything and everybody around her as if they were in an earthquake? Why does this happen? Who knows!?! My guess is that the director thought it looked "cool".
2.) This movies contains dozens of shots, in slow motion, of course, of birds showing up out of nowhere and flying off, and most annoyingly, of water dripping. This woman's apartment is constantly dripping water! CONSTANTLY! Logically, the place would probably fall apart with this many holes. To sum up this complaint, towards the end, and for absolutely no reason, the camera cuts to shots of water dripping, in slow motion, in reverse!! WHY!?! I have no idea! It has no relevance to anything, and once again, I'm betting it's because the director thought it looked "cool".
3.) One of the main characters says he became a priest to explain away holes in science. This doesn't make sense to me. I would think that going to church would be enough, but no, he has to go through the entire rigamarole of becoming a priest. I just don't buy it. Secondly, there are lots of plot holes, a few of which I will elaborate on below. For starters, when she first gets the stigmata, the scene ends with her laying unconscious, bleeding. Next, she's in the hospital. Who called the ambulance? Another one is towards the end, when the previously mentioned "scientific priest" character is talking to the spirit who is possessing the girl. He says, "Take me as your messenger!" Not a word for word quote, but you get the idea. His response? "You have no faith, only doubt!" So, because of this, he possesses an aetheist! An aetheist has no faith, far less then any scientific priest! And then, there's the fact that the object of this movie's conspiracy, this Lost Gospel (of St. Thomas, I believe) is available at local bookstores. The characters are willing to kill to supress this document, but you could walk down to a bookstore and buy it. Maybe this is supposed to take place in an alternate history, where it isn't wide known, but the movie never tries to tell us this, or to even hint that this is an alternate happening of that document's uncovering.
In closing, this movie is terrible to a spectacular degree. It is my arch-nemesis, which I feel the need to insult every chance I get. I loathe it. Final Grade: F