Even secular scholars have rejected the idea of Christianity borrowing from the ancient mysteries. The well-respected Sir Edward Evans-Pritchard writes in Theories of Primitive Religion that The evidence for this theory... is negligible.
The first real parallel of a dying and rising god does not appear until A.D. 150, more than a hundred years after the origin of Christianity. So if there was any influence of one on the other, it was the influence of the historical event of the New Testament (resurrection) on mythology, not the reverse. The only known account of a god surviving death that predates Christianity is the Egyptian cult god Osiris.
In this myth, Osiris is cut into fourteen pieces, scattered around Egypt, then reassembled and brought back to life by the goddess Isis. However, Osiris does not actually come back to physical life but becomes a member of a shadowy underworld... This is far different than Jesus' resurrection account where he was the gloriously risen Prince of life who was seen by others on earth before his ascension into heaven. –Dr. Norman Geisler.
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