Oh boo hoo, so you make the upped cost 1 less (or the unupped cost 1 more)...
Anyway, I think it might be a bit too biblical to fit in with the rest of the game, but I really like the concept!
I would make it gain +1/+0. Since it comes into play immortal AND has 7 health, that's not a major drawback.
What would be really cool: if the fallen ability could be played by either player (and that the player who plays it is in control of the Devil afterwards. Makes it more in line with the original fairytale =)
On a sidenote: you should make a series out of archangels!
I can just picture a Michael card that is a similar card, but has a Fahrenheit-like ability (or that has emblaze, I guess)