"Religion declined not because it was refuted, but because it became irrelevant, dull, oppressive, insipid." -- Abraham Joshua Heschel
Heschel was Jewish, and the only religion I feel qualified to speak about is Christianity. It was always irrelevant, dull, oppressive and insipid. It is declining now because it has been refuted. And/or because after 1000 years of torturing and killing everyone who disagreed with them, Christian leaders gradually have been forced to accept more open discussion of these things. Without the torturing and killing, would Christianity ever have spread so far to begin with? In other words: did people EVER really accept it, or did they act as if they accepted it, because -- torture and burning alive? We'll never know. Because torture and burning alive as punishment for questioning orthodoxy don't encourage people to go on record with their real opinions.
That is far from a brilliant insight on my part, it's quite simple and evident. And yet it's one of the simple and plain aspects of the history of Western civilization which still is rather rarely acknowledged. Whether the Christian authorities stopped torturing and killing simply because they lost the authority to do so, or because they actually became more tolerant and merciful on their own, they still very energetically push a lot of nonsense. Where they have stopped actively combating the natural sciences, they now often turn to combating those of us who are struggling to make some sort of sense of history. Heschel is Jewish, but his statement quoted at the beginning of this post could have come from any of a number of Christian theologians and theologically-inclined historians of Western civilization who energetically, full-time, propagate nonsense about the subjects they ostensibly teach. Religion became oppressive? It has been 200 years since the Inquisition tortured and killed anyone. Clearly, religion is less oppressive in Western society today than it was in the Middle Ages. Few things could be so clear. But a lot of people who are supposed to be teaching about the Middle Ages are doing all they can to make them less clear.
Apologetics makes historical writing worse. Some scholars who in previous ages would have concentrated on non-stop invention of nonsense about "spiritual realms" -- that is, worlds of make-believe -- now concentrate full-time on shamelessly distorting those earlier eras, on making them seem less crazy and horrid than they were. They'll say that the beauty of Medieval cathedrals reflects an extraordinary level of piety and religious fervor in the time they were built. I agree with them that the Cathedrals are beautiful, but I say that they reflect a time in the dominions of Catholicism when the Church was far and away the biggest patron of the arts, and for very many artists the only patron who could ever pay them. Cathedrals are magnificent because when they were built, they were the only opportunity for most artists to express themselves. Art in Medieval Europe for most people equaled Catholic art, not because everyone was a fervent Catholic but because Catholic art was the only art that was allowed. Art must have been an especial comfort in that dull, oppressive, insipid time.