That's over-simplified on many levels
The notion that the Dark Ages were purely a product of Christianity is a massive over-simplification on many levels.
The real problem there is that the relative safety and organization of the Roman Empire had collapsed, and society, trade, science collapsed with it. Christian or not, nobody had resources to do much more than flock around some castle and hope that the knights there can defend them from the raiders. They just didn't have the resources or manpower to support large cities with libraries and research centres and whatnot, and with the nearly complete collapse of trade the flow of ideas across Europe had halted too. And research doesn't work in a vaccuum.
The fall of Rome and the rise of Christianity also aren't as clear cut things as some people seem to assume. "OMG, Christianity killed the Roman Empire and brought the Dark Ages!" Not.
First of all, the Roman Empire had contained the seeds of its own destruction all along. The Imperial age had started with more than one example that might is right, and marching with the legions upon Rome is the perfectly legitimate way to get the throne. That's how Caesar did it, and Sula before him. They had no concept of even translation of power or who should inherit or who should designate the next emperor. (The Senate wasn't it. And it had been robbed of even most of its auctoritas, and certainly lacked imperium or potestas to start with.) The only right and justification in getting to the top was little more than "because I can."
Civil war raged half the time, and hundreds of thousands of its own soldiers died routinely. You know, soldiers who would have been more useful in defending the borders. In the third century crisis they often had several emperors in the same bloody year. And a literally bloody year it was when that happened.
Constantine (who also came to power in a civil war, btw) held about 400,000 soldiers as his personal guard just to deter potential usurpers, and that was a ludicrious percentage of the Empire's armies. He had pretty much depopulated the border garrisons just to keep his own throne safe.
In that sense, inventing a divine claim to the throne there was actually a step forward from "whose army is the biggest" system of previous centuries.
Barbarians had just become a bigger problem than ever too. See, previously Germans and Celts had been hunter-gatherers with small populations and poor equipment. Rome could deal with that. But then at the start of the current era, a plough was invented that worked in Germany. A population boom followed. Those tribes could now throw more and more people at Rome's borders, and soon they could pierce the defense in any point they wanted.
A second thing happened in Germany too: the invention of the horseshoe. While it doesn't get as much press as the stirrups, it raised the efficiency of gothic cavalry almost overnight. Rome's legions and equipment switched from an offensive thing, to just trying to defend.
Plague was an even bigger problem. At the peak of one of several plague outbursts, 5000 people per day died in Rome. (Though that would later be topped by Justinian's plague which killed 10,000 a day in Constantinople at its peak.) The economy and manpower of Rome pretty much imploded, to the effect where they couldn't sustain their legions on the border or their pay or their equipment.
In a couple of centuries, Rome went from well over 1 million inhabitants, to about 20,000 people living among acres upon acres of ruins and abandoned buildings. Think: a Fallout scenario.
But plague didn't just hit the economy and manpower, it hit morale hard. The rise of Christianity didn't _cause_ the decline of Rome. On the contrary, it was _caused_ by it. People lost faith in the old Gods who had failed to protect them, and turned to any esoteric eastern things instead. Not just Christianity but also stuff like the mysteries of Mithras.
Legions got to be 500 to 1000 people fortified somewhere defensively, down from about 6000. Soldiers' pay, also due to devaluing the coins, went down to bare subsistence levels, so enlistment also disappeared. The Empire had to enforce conscription (with penalties going at times all the way to burning at the stake for self-mutilation to escape conscription.) Tactics switched from offensive to trying to minimize casualties, because soldiers had become that hard to replace. Equipment fell back to chain and spears just because they couldn't afford to give them a segmented armour any more.
Etc.
And then came Justinian's plague, as Justinian's reconquest to Italy brought it there too. The economy of Italy pretty much just collapsed. The country side got too depopulated to support either any noteworthy cities or any noteworthy army. You can guess how bad it got when Italy was just rolled over by a tribe as primitive as the Longobards.
Not that in other places it was much better. The Franks weren't exactly intellectual giants when they overran Europe. Charlemagne was the first germanic monarch who wasn't illiterate and proud of it. Well, he still was illiterate, but he did try earnestly to learn to read and write, albeit with very limited success. In Britannia the plague pretty much wiped out their manpower and economy too, so they had to bring in the Angles and Saxons to keep the Picts in check. And then got overrun by them when they could no longer pay them.
And in a nutshell _that_ is how the Dark ages started. Blaming it only on Christianity is just ignorant. It was a general collapse of society on such a scale, that no religion could have caused or stopped.
