The theory that the story of Jesus was assembled from older pagan gods.
Interesting claims
Jesus as recycled pagan myth
Popularized by the film Zeitgeist (2007) and earlier writers like Gerald Massey, the claim holds that Christianity plagiarized Egyptian and Persian savior myths wholesale: virgin births, December 25th, twelve followers, crucifixion, resurrection.
Assessment: Nearly every specific parallel collapses on inspection. Horus was not born of a virgin or on December 25th; Mithra emerged from rock; "twelve disciples" lists are fabricated. The theory relies on sources that do not say what is claimed.
Documented
Real influence, different scale
Legitimate scholarship does document exchange: mystery religions and Christianity shared an era and vocabulary; ideas of dying-and-rising gods, baptism-like rites, and sacred meals existed across the Mediterranean. The December 25th date does likely borrow from Roman solar festivals. Influence is real; plagiarism of a checklist is not.
Assessment: The honest position is less dramatic than both the conspiracy and the total denial.
Stories
The list that never dies
Born on December 25th, twelve disciples, three-day resurrection, performed miracles: the checklist circulates as image macros and documentary narration, attributed to Horus, Mithras, Dionysus, Krishna, depending on the version. It is the most shared comparison in the history of religious argument.
Assessment: As folklore about religion, the list is a documented phenomenon. As claims about ancient texts, nearly every line fails on contact with the actual sources. Its immortality is a fact about the internet, not about Egypt.
Sources and further reading
Plutarch, De Iside et Osiride 12-19, the Osiris myth in Greek
Kersey Graves, The World's Sixteen Crucified Saviors, 1875
Zeitgeist: The Movie, part 1, 2007, the modern vector
Worth asking
If the parallels dissolve under scrutiny, why did we believe them instantly, and what does that reveal about what we wanted to be true?