The Guru. The one who has achieved the Param Padvi. The one who has Puran Jyot Parkash in His Hirda. The one who has won over Maya. The one who has merged in Akal Purakh and the one who has been authorized either by another Puran Brahamgiani or by Akal Purakh Himself to deliver GurParsaad to the masses. The most important thing to understand and truly believe is that a Puran Sant, a Puran Brahamgiani, a SatGur is a living God on this Earth.