Father of Understanding
The Father of Understanding, is a being (symbolic or otherwise) of whom the Templars pledge allegiance to and ask for guidance. During the Crusades, the Templars would usually sign the name before their own in letters they wrote. During the Renaissance, Templars would often ask him for guidance as a greeting or at the end of a meeting:
"May the Father of Understanding guide us"
And in an e-mail, Alan Rikkin writes to Warren Vidic: "May The Father of Understanding guide you to success", meaning that the Templars still use the saying.
Theories
While many speculations and theories have been put forth regarding the identity who the Father of Understanding is, ranging from Cain to the "Those Who Came Before" aleatory of Satan, to the Templar's founder; it could also, likely be a reference to the actual trait of understanding and acceptance, thus making the Father of Understanding a concept rather than any actual being (Such as Father Time and Mother Earth). In either case, it could simply represent the Templars' longing that their harsh means of attaining peace be understood as not as it appears, but as a necessary means of achieving a greatness beyond measure; "absolute peace".