I'm very into the subject of alternate history. I've actually read a few things about those two things.
The topic of this thread is:
If you could go back in time would you save Jesus Christ from getting crucified? Why or why not? If yes, then how would this effect the spread of Christanity throughout Europe?
I have a book published by Putnam titled "The Collected What If?" which has "What If" and "What If 2," and are essays regarding alternate histories. One of these includes an essay on how history would be different if Jesus Christ was never crucified by Pontius Pilate.
What happens is Pilate gives Jesus a pardon. More than that, he gives Jesus protection by Roman soldiers who serve as bodyguards. This is because Jesus teaches forgiveness and submission, and the Roman occupiers of Jerusalem twist his teachings to forgiveness and submission towards Rome, and highlights quite literally that the Jews should "give unto Caesar..."
Still, there are elements of Jews who want to overthrow the Roman occupiers, and the Zealots pursue their revolt in Palestine, which is crushed by the Roman legions. Rome then does a siege on Jerusalem, breaks into the city, destroys the Temple, and occupies the city. However, the Roman soldiers are ordered to keep Jesus safe so he may continue his preaching of nonviolence.
Jesus lives on until the age of 97. He dies quietly in his sleep. He is buried according to Jewish tradition. Three days later his body disappears, and there are sightings of him all over the world.
230 years later Constatine is Emperor of Rome, for he decided to stay rather than move to Byzantium as his Greeks advisers would have him do. Christianity is the state religion of Rome, and because the Empire is not split the Germanic barbarians are able to be repelled. Christianity is able to spread across Europe as Rome spreads it's civilization, strong in the West but clashes with the Muslims who attacks Rome on in the Near East. Constantine considers rebuilding the Temple of Solomon in Jerusalem, but decides instead to rebuild it in the capital, and christen it the Temple of Constantine.
Also, would you assassinate Adolf Hitler and stop the Nazi regime in Germany? Why or why not? If yes, then how would this effect the World War II era?
One of the best alt history stories I've ever read regarding stopping WW2 was about this group who traveled back in time to aid the German government and military during the Great War. Because Germany was able to invade France and occupy it, Germany never suffered defeat and therefore never experienced the depression of the '20's that was the catalyst for the German government's downfall that allowed Hitler to seize power.
However, years after Germany's victory, there were French men and women who saw themselves oppressed by Germany's government. French political radicals held rallies in the cafes, and advocated "Gallic Superiority" over the racially inferior "Huns" who had occupied them. One of the time travelers sees the parallels of this and is quite horrified by what may come of it.