A certain group of angels have decided, based on their interpretation of the will of God, to bring about the end of human life. These angels are known as the Fallen. Led by the angel Lilith, they have traveled to Earth and taken human form. They then used their angelic powers to take over the government and its army of soldiers, instituting an authoritarian regime. They are planning to somehow use the starship Leviathan to bring about Armageddon.
I hope the fireball isn't headed this way
You play the role of Malachi, a loyal angel. Guess what your job is? That's right, stop the bad guys. There is a nascent resistance to the authoritarian regime forming on Earth. Malachi must find a way to work with the resistance and stop the Fallen.
Old fashioned shotgun diplomacy
I have to admit that the back story is original. Sure, on one hand it's just another excuse to blow stuff up, but on the other hand it ties together two seemingly unrelated items: religion and technological future. It's an interesting twist.