Re-agree: Apostle's Contract

From Chinatown Km: Once SA, a man who refuses to believe in fantasy, sees a cute girl floating from the sky. His world is quickly broken, and he is transferred to the world of "Christ" and "Apostles". However, the turn is that these "apostles" are fighting for their lives (and they are girls, not to mention). Who are these "apostles" and for what they fight?