How would history have been different if Constantine the Great never became emperor? Or if he lost the Battle of Milvian Bridge to Maxentius? Or if he and Licinius never issued the Edict of Milan? Would another emperor have legalized Christianity? Could it sill have become a major western religion or would paganism remained dominant? Would the Roman Empire collapsed sooner or lasted longer? Without Christianity as a major player would Islam devolped?
Well, Keanu Reeves’s career wouldn’t have stalled…
All my replies are theologically, not historically grounded.
Basically, I think Trinitarian C’nity would have still emerged as the dominant Western world religion- perhaps in better form by continuing to rely on winning popular hearts & minds rather than imposition by the establishment. Arian C’nity,
Gnosticism & Roman paganism would have lingered on longer as rivals, but eventually fallen out of mass favor. Christendom may not have established anti-Semitism under the guise of anti-Judaizing. The primacy of the Roman See & the resultant power struggles with the Eastern bishops would have been less likely.
A united non-Imperialized catholic orthodox Church would have deepened its roots among the Arabs, and as Harry Turtledove envisioned, we may be remembering fondly St. Mahomet, Apostle to the Arabs, who coined of “There is no Allah but Allah and Jesus is His Son.”
In which book was this?
Assuming Constantine loses the Battle of the Milvian Bridge, Maxentius then would have defeated three Caesars/Augusi (Severus and Galerius had already failed to disloge him from his nearly impregnable position at Rome). The Roman world would be divided between him and Licinius, and it is likely at some point the two would meet in battle, and I believe Licinius–with his greater resources in the East–would win. By this time, Rome was the capital of the empire in name only.
As to the effect on Christianity, even Diocletian by the end realized that persecuting the Christians was pointless, and Galerius had issued an Edict of Tolerance toward the Christians a few years before the Edict of Milan. But Constantine’s edict went further by restoring seixed property/etc. to the Christians; in effect legitimizing it and displacing the native Roman paganism to make it the de facto state religion. Constantine proceeded gradually in establishing the new religion, but it is clear he saw in the Christian religion a potential unifying effect that the pagan religion no longer sustained. I suspect some later emperor would also realize this, and like Constantine would see some political advantage in fostering the religion.
Paganism wasn’t really dominant by the time of Constantine, so I doubt it would have lasted even without Constantine’s accession, but there were other monotheistic religions which could have stepped in if Constantine hadn’t chosen Christianity–Mithraism is an obvious example, since it remained relatively popular with the army and was even encouraged by emperors prior to Constantine. The problems with Mithraism are outlined in the link, and it may be a bias against the “winners” in the battle for religious supremacy, but Christianity appears to have been uniquely positioned among religions to expand into the role Constantine applied it. In short, the religion likely would have continued to grow under the empire, and likely still become a major world religion. Whether or not Constantine himself was devout (he likely was, although perhaps not in the way most of us understand modern Christian devotion), he used Christianity to his own, imperial ends, and I’d expect any shrewd emperor would have done the same. Similarly, I’d expect some future emperor to eventually move the capital as Constantine did, and likely place it near Istanbul (the trend toward the wealth of East had begun with Diocletian, and Rome was constantly at war in Dacia and Persia), and–as with the adoption of Christianity–would attribute the move to the necessity of historical forces.
Christianity–Gibbon’s analysis aside–was likely not a major factor in the fall of the Western empire. It is clear the transformation from the late empire to the Middle Ages was a gradual process over many centuries. We see, for example, the seeds of Feudalism in the third and fourth century Roman world (e.g. to sustain the tax system, many occupations were by imperial fiat made hereditary; as imperial security faded, farmers became more dependent on the local landowner). If anything, Rome itself may have fallen sooner if not for Christianity (a religion many ‘barbarians’ also adopted).