If your faith is strong, why are you worried about demons?
The amount of faith (i.e., "strong" or not) is not the issue here. The issue is basically if you "invite" demons into your life, you will get them. The point is to not invite demons into your life.
And thus, the warning for all others to be aware of that.
If these things worked anywhere near the way you are implying, we'd be up to your necks in Anti-Christs and Demon-spawn.
Ummmm..., from the looks of it in our society, I would say that's the way it is ... LOL ...
And if that were even remotely true, Buffy the Vampire Slayer would have been a documentary instead of a campy TV series.