Zeitgeist: The Movie is three unrelated arguments packaged as one film, and only Part One, "The Greatest Story Ever Told," is a religious claim. Released free online in 2007 by Peter Joseph, it opens by asserting that ancient civilizations from Egypt to India already worshipped a sun god born of a virgin on 25 December, dead and resurrected after three days, with twelve followers, and that Jesus is simply the latest costume for this recurring solar myth, with Horus, Mithras and Krishna offered as the clearest predecessors.
None of that argument originated with Peter Joseph. Its specific claim list traces to D.M. Murdock, writing as Acharya S, whose books The Christ Conspiracy (1999) and Suns of God (2004) assembled the parallel list Part One recites almost point for point; Joseph collaborated with Murdock directly on a companion volume, The ZEITGEIST Sourcebook, defending the film's claims after release. And Murdock's own Horus material is itself a restatement of Gerald Massey's Ancient Egypt: The Light of the World (1907), a self-taught nineteenth-century writer whose etymological method no Egyptologist of his own generation, or since, accepted.
So the film is best read as the last and largest link in a century-long chain, Massey to Murdock to a documentary that reached an audience of millions, rather than as an independent piece of research. That chain is recorded here plainly, on Massey's and Murdock's own pages, rather than left implied.