Shadow, this is a great concept. I got a sense of the setting and story just from your short description. I especially like the historical/mythological tie-ins: Knights Templar, Prometheus, etc. Those extra little bits are what make a movie stand out.
That said, I've got one caveat: the proof is in the pudding. It's one thing to have a good idea. It's quite another to translate that idea onto paper. I'm not trying to discourage you at all. What I'm saying is that, since you're new to writing, you really need to get some practice under your belt. Read some scripts from movies you love. Write a few little one-acts just for practice. You only get good at writing by writing. I think you've demonstrated that you've got the talent for imagination already, so don't worry about that aspect. Just write, write, write until you've found your own unique voice and style.
On a more specific note, I agree with adrian: keep it simple. An audience can identify with 20 heroes much more easily than 200. That allows you more freedom for characterization as well.
One thing that occurred to me: since this is a somewhat post-apocalyptic world, a great deal of the "dead" technology would still be lying around, waiting to be picked up. You'd have a lost-and-found society, at least in part. It would be kinda cool if the MC (or other characters) incorporated this "found tech" alongside their more medieval armaments. (Just a suggestion. I'm not trying to hijack your idea, I swear

)
This is an exciting concept, and I hope to see some work from you in the future!