You know those pivotal moments in history (the ones that Doctor Who isn't allowed to change)? Well just imagine that we lived in a universe where one of those crucial moments went differently. You can play this game with any pivotal moment you like (it's an amusing way to while away a winter's evening).
The crucial event I would like to imagine going differently is the Council of Nicaea in 325 CE. Picture the scene... Diocletian's reign of terror has only recently ceased. Suddenly the early Christians are no longer united in being persecuted - now they can turn on each other and start weeding out the heretics. Add to the mix the Emperor Constantine, who turns up three days into the Council of Nicaea and is presented with a bundle of papers representing the theological deliberations of the bishops before his arrival, which he promptly chucks on the nearest brazier. One of the most important arguments that is had at the Council of Niceaea is the controversy over the Trinity and the Arian heresy. The Arian heresy is the belief that Jesus was the Son of God from the moment of his conception, not from the beginning of time as Trinitarian orthodoxy insists. It was revived after the Reformation by Faustus Socinus (founder of the Socinian Brethren, which eventually became the Unitarian church in Transylvania).
The importance of the Arian heresy is that it makes Jesus either semi-divine, or divine by adoption, or divine by birth (rather than divine since the beginning of time). If this is the theological position one adopts, it means that he ceases to be seen as the sole means of access to the "Father" (the Divine Source in Neoplatonic terminology), because if he is a son of God, rather than the Son of God, then there are other sons and daughters. And this quickly leads to Unitarianism - the belief that the Divine is One and can be accessed by reason and intuition, and does not require revelation to be known. That's not to suggest we can fully know the nature of the Divine, but we can see it reflected in the world around us, in other people, and the beauty of the universe. It also means that if we are all children of God, then we all have the potential to develop our inner "Christ" / Messiah / Buddha / Enlightened One.
So, let's imagine that the Arian heresy had won out at the Council of Nicaea. Perhaps the word heresy would still have its original meaning of a school of thought, or a choice (from the Greek haeresis).
There would have been no need to convert most of Europe by force - because, as Jesus said in John ch. 14, other peoples have their own religions (including Paganism) by which the Divine makes itself known.
The doctrine of penal substitution (the idea that Jesus' death was a substitute sacrifice for humanity's sins) might never have arisen (it was formalised in 1098 by Anselm).
The story of Jesus' resurrection might have been seen as a triumph over death, or perhaps eventually as an allegory of psychological transformation.
Islam might not have developed as a distinct religion (it is possible that it evolved out of an Arian group - it certainly holds a similar view of Christ). There would have been no Crusades, because no need to wrest the control of Israel from the Muslims, because they would have been seen as fellow believers.
The Jews might not have been so viciously persecuted (Unitarian churches have long had good relations with Judaism).
The Reformation might have been very different: Calvin couldn't appropriate Anselm's penal substitution theology, because it hadn't been written. Nor would he be able to have Servetus burnt at the stake for his Arianism. Indeed, Servetus might have been a major mover and shaker in the Reformation.
Tolerance of other religions would have been much greater, which would have made the imperialist and colonialist activities of Europe very different. There would have have been no need for evangelical Christianity, because the "good news" that Jesus was killed so you didn't have to be fried for eternity would never have been invented.
I wonder if there really are multiple universes where different choices were made at pivotal historical moments? It would be so interesting to visit them.
If my alternative universe is too tame, try Sannion's vision of the Roman empire with zombies.
Fundamentalist Christianity: the belief that a cosmic Jewish zombie who was his own father can make you live forever if you symbolically eat his flesh and telepathically tell him that you accept him as your master, so that he can remove from your soul an evil force that is present in humanity because a woman made out of a rib was tricked into eating a magic apple by a talking snake.