There is a certain genre of fiction that I am particularly fond of: Alternate History. Basically, it's as its name implies, a narrative involving the history of the world if something in history had happened differently. For an example, take the lack of Christianity. Whoa. Needless to say, that change would be immense. I've been thinking about the incredible potential of such a change.
While not going into how, the differences of such a world would be enoromous. For one thing, one could argue that the Roman Empire would not have been so weak against German invaders because of Christianity's weakening of traditional Roman military values with a focus of a kingdom in heaven. However, it could be said that Christianity, being a more advanced religion than most pagan ones, fulfills a certain internal need, so Roman religion might eventually evolve to involve these things. I would imagine that it evolve into something like the Greco-Roman Mystery Religions, especially the ones involving Dionysus and the Egytian Osiris.
I would imagine that without any Christianity, there may not have been the Dark Ages for Europe, as the Roman Empire would have remained intact to an extent, and intellectual development could have continued as they had for the pas thousand years. That could have made the modern world extremely advanced. The defining feature, I think, though, of this Modern Roman society would be its morals. It would not have any concept of the Christian idea of 'love one another' or a 'kingdom in heaven'. It would have been extremely unequal and hedonistic (pleasure loving). All in all, it would be fascinating to see, I think.
Even more fascinating though, would be how the Germanic (and by extension, Norse) races would have developed without Christianity. I can think that a society that developed that already valued glory in battle and violence (remember Beowulf?) would have that same need to have a more advanced religion as the Romans. This could lead to some interesting scenarios. I think that perhaps the Germanic tribes would have eventually been unified into a Nazi-like state worship, emphasizing above all the the glory of dying for the nation (I could see a leader claiming to be Odin incarnate or something). Maybe instead it would be less organized and would resemble extremist Islam with the concept of Jihad. The morals of this society would be drastically different than it is in our society.
I am not sayin that my ideas definitively would happen. These are just my ideas. What do you think? What do you think would happen if there were no Christianity in the world? To the Romans? To the Germans? To the Middle Easterns? Feel free to speculate.