I mean what happened did set us back astronomically.
And it was because of a religious movement.
I'm not justifying blaming Christians for everything but this particular series of events was definitely their bad.
I'd say it more had to do with the fall of Roman Empire than just Christianity alone. A lot of things like concrete were forgotten after the fall of the roman empire because it got too expensive to make, so they stopped working with it, one generation later it was forgotten. Thankfully it was rediscovered. Another thing to blame was the lack of printing since everything was hand written back them.
Another thing to consider is that they had schools and paper is a fragile material since it's prone to water and fire damage, plus it decays pretty quickly. Plus you have barbarians and cultists (of any religion) running about damaging libraries and just destroying stuff for the hell of it.
Even in the Egyptian, Greek, and Roman periods you also had fewer educated people. Most people were pretty much born as laborers. Only those born into wealth had time and money to become educated. Combine that with the lack of printing, the weakness of paper, it makes sense that we lost a lot of knowledge. Plus it took forever for news and information to spread around back then too. Religion was a big thing back in those days just as much as it was in the Dark ages. There is even evidence of other religions destroying stuff back then too. Like Göbekli Tepe which was built in the Neolithic period(think cave people, mammoths, etc):
It was preserved under the sand which has lead archeologists to believe that it was buried by humans. Had it been buried naturally, it would have been more eroded.
I will say though that the dark ages advanced military technology and fashion. To think that we would be wearing togas or walking around naked right now.
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