Honestly not sure how much I'd credit religion there. It definitely plays a role - with Narnia in particular - but it's also true that a lot of the fantasy currently quite popular has fairly godless underpinnings, and to which some religious figures have reacted very hostilely to (the controversy over Harry Potter).
- You can, if you choose, write the exact same story and then create two versions, one which is fantasy and one which is science fiction, simply by word choice.
"TrekBBS Poster, I am you - from the future. I used an incantation to open a portal between my present and yours. Prophesy has foretold that dragons will attack this world unless I gift you our magic."
Science Fiction wrote:
"TrekBBS Poster, I am you - from the future. I used a temporal translocator to open a dimensional rift between my present and yours. Psychohistory has predicted that alien monsters will attack this world unless I gift you our science."
They're both terrible, incoherent stories, obviously, but you get the idea. I would have to change more than I did to turn them into romances or thrillers.
Um...I don't know if this has already been suggested earlier in this thread, but it is not the plausibility or even the possibility of the imaginary science or technology in the work of science fiction that differentiates it from most fantasy. What makes it science fiction rather than fantasy is whether, inside the fictional world, those fictional characters consider it a form of science.
Science and magic are NOT the same thing. Even implausible or impossible science is not the same thing as magic. For example, magic, to function, often relies on the personal skill, power, concentration, etc, of the user. Scientific experiments, however, once understood, and with a good, clear instruction manual, can be reproduced, more or less, by any shmoe off the street. Science is provable and repeatable, given all the same variables.
There are also fantasy novels where magic is all of these things happen. In fact I've read one which focuses on a physicist teasing out the scientific properties of thaumaturgons, an elemental particle that is responsible for magic.
It's obviously not universally
true of magic, obviously, but a fantasy novel that treats acts of magic as provable and repeatable and consistent in the manner of science does not make it a science fiction novel.
And on the other hand there are scientific novels where what is going on surpasses the comprehension of the characters. Solaris
is mostly about the entire scientific process being confounded by the unpredictability of a single planet.