This buys into the fallacious Great Man Theory of history.
Constantine didn't make Christianity the official religion of the Roman Empire; he ended government-sponsored persecution of Christians in 313, and while he himself was sincere in his conversion, most of the Patrician class remained Pagan, and even the Emperor can't piss off powerful people too much. Christianity didn't become the official religion until 380, under Emperor Theodosius. But it's not like Theodosius forced Christianity on a Pagan populace; Christianity had been growing in the empire for centuries, initially mostly among the poor and slaves. Poor people and slaves have less ability to change things than the rich, but at some point they get big enough that actively opposing them leads to riots and destruction, and you don't want that.
So Rome would have become Christian anyway, no matter who was emperor. Different people holding the office would have resulted in it happening a little differently, but the overall trend was towards Christianity replacing Paganism.
Christianity also didn't result in the fall of the Western Roman Empire. Setting aside that the Roman Empire continued in the east for another millennium, the western part of the empire collapsed due to economic and political forces. It's the same instability that allowed Julius Caesar to become dictator for life, 44 years before Jesus was even born. The empire was totally dysfunctional, so when a populist marched an army in and ordered the Patricians to elect him permanent dictator, they did what he said. But the systemic issues affecting the empire could only be held in check for so long, and eventually the western part came crumbling down.
But it's also incorrect to assert that no new developments took place during the Middle Ages. Off the top of my head, that's when lowercase letters and spaces between words were invented. Plough design also improved a lot during the Middle Ages, allowing more land to be cultivated more effectively.
And one last thing. You say "That shit set civilization back by centuries." Why would the events in Rome have any bearing on the developments of civilisation in Ethiopia, Iran, Arabia, India, China, Cahokia, Mexico, or Peru?