From the back of the book:
Peace of Man
As a Magus you wield magical might beyond humanity's comprehension. You command the wind to blow, the sea to part, the forest to wither. You are powerful indeed. But what is the value of worldly power if you must pay for it with your eternal soul? In the eyes of the Divine only your virtue is worthy, only your soul is sacred. The Reward cares not for your life's arcane conquests. At final judgement, before Heaven's gate, only your pure soul will save you. What salvation will your base power offer then?
Peace of God
Pax Dei is an Ars Magica Third Edition sourcebook that invokes the glory and majesty of the Divine Realm in Mythic Europe. Player and Storyguide alike are made privy to the Church's worldly designs, and are offered brilliant insight into the Reward itself. Not only may you pursue Divine will, but you may now reconcile your magic and piety to rise above even the Order of Hermes.