The ending scene of the Martyrs is one of the most impactful scenes in cinema, let alone the horror genre. A flayed, but still somehow-still-alive Lucie whispers something into the ears of the Mademoiselle. What did she say? This is one of the most widely contested plot points of the entire film.
To that, there are only four logical choices:
There is an afterlife akin to heaven and it's awesome!
There is an afterlife and it's like hell, so horrible!
There is an afterlife, but it is entirely dependent on the person :/
There is no afterlife, life is pointless. Everything sucks.
Yeah, we're doing something akin to Pascal's wager here. Here’s what we know of Mademoiselle's reaction to guide our process of elimination:
She looks genuinely disturbed/shocked while Anna tells her.
She tells her assistant to "Keep Doubting"
She kills herself.
Let's start with 1. If she learns that there is an afterlife and it is wonderful, she might be keen to get there sooner, but then why would she want her assistant to keep doubting? Surely, she wouldn't care if he follows suit if she says, "It's wonderful and I'm going to go! Peace!" We can eliminate this choice.
It's the reverse logic for 2. If there is an afterlife and it's horrible - then why would the Mademoiselle want to go there? She would instead want to delay the inevitable as long as possible. In addition, if there is a terrible afterlife she might be keener to tell her assistant, simply because shared fear is always easier to handle. We can eliminate this as well.
Now let's look at 3. This could be the case, but again we can assume that Mademoiselle has done so many cruel deeds she wouldn't want to go there. If she feels like she is going to heaven, or something equally pleasant, it's such an uncertain notion I don't think she'd want to gamble by taking her own life. She certainly wouldn't need to tell her assistant to keep doubting. She should easily say "There's something after all" and then die; if she was so eager to get to a pleasant after life. Or, she'd give a warning about making the right choices. I think we can eliminate this one.
That leaves the last possibility, 4. This is the one that makes the most logical sense. If there is no afterlife, then Mademoiselle's entire work has been in vain. She has killed people, children even, and has invested her entire career to this singular moment. Not only is there nothing else to strive for, but there is nothing to look forward after her life's journey is complete. It's a depressing notion that would honestly make anyone want to check out early.
What's the point in living with that knowledge?
In addition, if you did have that knowledge and someone asked you, would you truly want to tell them the bitter truth? You would want them to, as she solemnly says, "keep doubting." It also explains her reaction when Anna tells her. Of course, this opens the possibility that more women and children will just keep getting tortured to find out what she was told and took to the grave with her. However, it confirms life is just pointlessly cruel.
Pretty depressing stuff, but at least it's an answer.