Most fundies believe a couple of things about salvation and damnation:
- Most people are going to hell.
- Children who die before reaching a (vague) age of responsibility will go to heaven.
This is made pretty explicit in the Left Behind books: all of the children below some age (roughly the age of puberty) are Raptured. The vast majority of persons above that age aren’t.
I doubt that either of these things is official doctrine in any conservative Christian denomination, but by and large, this is what most conservative Christians believe anyway.
They also pretty overwhelmingly believe:
- The fetus is a person, starting from when the sperm bonks into the egg.
So:
P(fetal person goes to heaven|fetus is aborted) = 1.
P(fetal person goes to hell|fetus is aborted) = 0.
But:
P(person goes to heaven|person is born) < .5.
P(fetal person goes to hell|person is born) > .5.
This is especially true because, in the fundie worldview, the sort of woman that would abort her baby is a baaaaaaaaaaaad woman, so if she carried her child to term, that child wouldn’t grow up as a real, true Christian, and would be less likely than average to find salvation.
So when such a woman aborts her fetus, she’s actually doing the fetal person a favor, because she’s taken its chances of an eternity in heaven from dubious to guaranteed.
What more could you do for someone than that?
The point, obviously, isn’t that the conclusion is true. It’s that the conclusion is ridiculous. But since the logic is airtight (at least, I’m not seeing where there could be a hole in it), that means at least one of the premises is absurd. Specifically, at least one of the first two, since premises (1) and (2) similarly justify mass murder of young heathen children without any help from (3).