The Ain Soph, loosely translated as the Understanding of God, was a concept from Jewish Qabalah. Leaders of the Service interpreted it as a single word which could be used to decipher any code and reveal any secret on Earth; they searched for the Ain Soph, but none after the 17th century were serious in their attempts.
The Godfather of Faction Paradox at the Gregorian Compact hinted that the Faction possessed the Ain Soph, citing it as his tool for crossing from the War in Heaven to their world. (COMIC: Bêtes Noires & Dark Horses)