For my write-up today on David Marshall's Jesus and the Religions of Man, I'll use as my starting point something that Marshall says on page 9:
"The pattern of a hero who descends into the earth to rescue those trapped in darkness, facing death to deliver those without hope, appears again and again in both ancient and modern mythology. Orpheus crossed the River Styx and entered Hades in search of his lost love. Miao Shan, a patron saint of Chinese Buddhism, was murdered by her jealous father, but when banished to the underworld, chains fell off and captives went free. Egyptian civilization traced its origins to the time Osiris, god of the Nile, was killed by a jealous Set, god of shriveled harvests, and his sister-wife Isis brought him back to life."
Before this passage, Marshall talks about Plato's Myth of the Cave, in which a man escapes a cave, observes the outside world, and is killed when he goes back into the cave and tells the people there about what he saw. Soon after the passage, Marshall refers to real-life examples of people who suffered to bring about a rebirth in society: Gandhi, Martin Luther King, Nelson Mandela, and Alexander Solzhenitsyn.
Marshall is setting the stage in this passage for the rest of his book, the aim of which is to discover the truth (which he believes is Christianity) amidst so many religions. Marshall asks on page 10: "What relation does the Christian story bear to other solutions to the problem of the cave?...Should the life of Jesus as it appears in the Gospels be classified among mythologies like Osiris and Miao Shan?" You'd think from this that Marshall seeks to differentiate the Gospels from other myths that present a savior-figure descending into suffering or death and bringing forth new life. Marshall goes that route on page 124 of The Truth Behind the New Atheism, where he appeals to arguments by Ronald Nash and Glenn Miller to refute the charge that Christians borrowed from pagan myths. (Note: There is actually much more to the Glenn Miller article than I link to, and Miller links to that at the bottom of his article, and then he links to more at the bottom of that article.) According to Nash, many have anachronistically projected later material (about, say, mystery cults) onto the first century to argue that Christianity is a copy-cat religion, and that is bad scholarship.
But in other places of The Truth Behind the New Atheism, and also in Jesus and the Religions of Man, Marshall treats Jesus as the fulfillment of pagan cultures, and he avers that there are Christian themes that are a part of our collective unconscious. C.S. Lewis went this route in that he saw the story of Osiris and Plato's Myth of the Cave as foreshadowing Christ. The view of Marshall and Lewis may be that God has implanted in humanity pieces of the truth about Christ, and that is why so many cultures (past and present) have elements that overlap with Christianity (i.e., rebirth).
I think that many anthropologists would agree with Marshall that cultural similarities do not necessarily indicate that one culture borrowed from another one. My hunch is that the reason that death and rebirth appears in so many cultures is that many places have winter and spring, which is a natural cycle of death and rebirth. The story of Baal, for instance, pertains to the natural cycle and the hope for agricultural fertility after a period of barrenness. Also, as Marshall notes, a new birth coming out of suffering occurs in the realm of real life, as we see with Gandhi, King, Mandela, etc. Maybe Christianity influenced that, in some way. Or perhaps Christianity is just one more example of people believing that life can come out of death, and of that occurring (in some manner).
An atheist may not find Marshall convincing, for the fact that a theme appears across so many cultures does not prove that one of the religions that has that theme (Christianity) is true. That could be why Marshall tries to use other arguments to support the historical accuracy of the Gospels. There is something cozy, though, about a motif that appears in so many cultures being historically-true and edifying. Is there a way for that to be the case without conservative Christianity having to be true? Marshall states on page 11 of Jesus and the Religions of Man: "Many hold that all these tales are projections of truth from the collective unconscious, the thousand faces of a universal heroic archetype we are called to become: Christ conscious, self-realized humanity." My problem with this is that it implies that I have to be a savior, when I'm more comfortable with someone else rescuing me. That's not to say, however, that I shouldn't do my part to help others.