WordNet (r) 3.0 (2006):

guiding light
    n 1: a celebrity who is an inspiration to others; "he was host
         to a large gathering of luminaries" [syn: luminary,
         leading light, guiding light, notable, notability]

Moby Thesaurus II by Grady Ward, 1.0:

24 Moby Thesaurus words for "guiding light":
   ambition, aspiration, basis, calling, cause, consideration, goal,
   ground, guiding star, ideal, inspiration, intention, lodestar,
   mainspring, matter, motive, principle, reason, sake, score, source,
   spring, ulterior motive, vocation