Close. This was the downfall in popular belief.
The real cause was of course the victory of Christianism over Rationalism, which caused hundreds of years of dark ages.
Do you think the Roman empire was notably rational, and that if Constantine hadn't converted to Christianity it would have been more rational than it was under Christianity? That seems highly not obvious to me, and I would be interested in any evidence you have.
That's what we learned in our history lessons.
The roman empire was not build upon military, technological or social achievements, but on a strong organizational and legal structural framework, the res publica. A rational framework.
Which was based on philosophical and political advantages (still a monarchic slave economy), but mostly it was a stable and written groundwork. Different to the US legal system btw, which bends fore-, backwards and even more backwards all the time, depending on your social and economical status.
The allowance and then even the adoption of Christianity allowed the Christians to refuse to pay the Fiscus Judaicus. https://en.wikipedia.org/wiki/Edict_of_Milan
Before, their refusal with their continuous insurgencies led to drastic persecutions, and now this principal to honor the res publica was just abolished, the roman state gave up on the martyrs. Monotheism led to sanctioning of completely irrational stories as told in the bible and various other lunatics, in contrast to the more rational polytheist societies.
I wonder whether we are using the terms "rational" and "rationalism" in different ways.
To me, "rationalism" means one of two things. When talking about the history of philosophy, it's the opposite of "empiricism", the idea that we can know important things by means of pure reason without observation of the world around us. When not talking about the history of philosophy, it means something like "trying to believe things to whatever extent there are evidence and reasoned arguments supporting them, and no further" (notable subtype 1: "rationalist" used to be a common term for a particular sort of atheist; notable subtype 2: "rationalist" these days often means "person who hangs out on the Less Wrong website").
But you're talking about "a strong organizational and legal structural framework", which is the sort of thing that might be designed rationally or that highly rational people might turn out to like, but having one versus not having one doesn't seem like the sort of thing I would want to call "rational" or "rationalism", and it doesn't seem particularly opposed to Christianity as such.
When the Roman empire embraced Christianity it didn't abandon the idea of the "res publica", or stop having an elaborate code of law. The only specific thing you point to is that it stopped imposing extra taxes on Christians and murdering them when they wouldn't pay, which seems to me a good thing however wrong Christians are (I agree that they are very wrong), and not obviously "irrational" in any sense I recognize.
I do not see how the stories in the Bible are any more irrational than the stories told in the Roman polytheist traditions about their gods. Could you explain?
Kind of the reason Christianity got a foothold is that belief in the traditional Greco-Roman gods was dropping in late antiquity. Lots of new religions were beginning to pop up -- Christianity, the cult of Isis (which was based on a Egyptian goddess but the way they worshiped her was new), the cult of Mithras. At one way, the death of traditional paganism can be seen as a win for rationality, although subsequent events suggests that getting rid of religion entirely is difficult because new religions tend to fill the vacuum left by the old making no net increase in overall rationality.
https://en.wikipedia.org/wiki/Altar_of_Victory#Removal