The Last Starship From Earth. In the present (novel was published in the late 60's), we have an advanced civilization, because the Dark Ages never happened. An excerpt from Lincoln's "Johannesburg Address" calling for nuclear disarmament was included. A theocracy was in place, the Pope was a computer, and Hell was a frozen planet that we would banish heretics to aboard a functioning starship. And Christianity's symbol was the crossbow, since that was what Jesus was killed by at the Battle of Masada at about the age of 80. A heretic is sent to Hell which turns out to be a nice place filled with people who want to bring down the theocracy, so they send him back in time to change history, which he does by faking Jesus' death on the cross and then sends him back to Hell instead of himself. To his horror, he has created
our form of Christianity resulting in our low level of technology and culture.
Those who fail to learn the lessons of science fiction are doomed to live them.
On another note, Suds, a mathematician on CompuServe who would basically support the ideas of word-magic, once posted something that actually made sense. It doesn't matter whether Christianity is true or not. Because everybody believed it to be true and acted accordingly, it had the same influence on western civilization that it would have had if it were actually true.
Christianity started as a mystery religion and had to have the requisite mythic elements. Which means that it had to have the death and resurrection regardless of what actually did or did not happen.