Explaining why Christianity's pathologies lay dormant for around 600 years or so is not in fact very difficult. It was temporarily de-Sophized due to breakdown in communication.
Sophism relies on some level of sophistication to work; after Rome fell, sophistication also fell and essentially everyone clustered at the grug-brain part of the midwit curve, incapable of understanding why they may want to take Sophist points in the least seriously.
Meanwhile, mortals' natural desire to be holy instead of unholy drove them to pervert Church teachings toward good. "Accidentally" misunderstanding that they're still supposed to honour the winter solstice, for example. With the Pope unable to tell them they're being heretical due to technological limitations, nothing stopped them from using Christianity in a relatively healthy way.
Unfortunately this did not remove Sophism from the Bible. The book remained relatively well-preserved. Oops. With the library at Toledo liberated, exposing Christians directly to Sophist texts, and sophistication rising again, the disease was re-activated.
Problem: Christianity is nearly Atheism, and Atheism is Gnosticism. It is not surprising that Christianity constantly spawns heresies which take its evil theology to its logical conclusion. Christianity tells you reject nearly everything holy and accept only the tiniest fraction of holiness.
"It's all demons!" Why? Because Sophists can use this to control you. They interpret which holiness is holy this week, and thus how you "must" behave.
Demons and devils can then take more Sophism and make the Christian reject even the tiniest fraction of the sacred they were allowed to keep. It's much easier than making Christians reject the original rejection.
Christianity is indeed the problem in the first place. And in any case, returning to Christianity is foolhardy in the extreme; it has already flagrantly proven its political inferiority to this particular heresy. If you rewind the tape of history all that happens is you get to watch the show a second time. Generally faster, because the players already know how it ends.
Christians, you had one job. It turned out you were working for the other side.
P.S. The reason Gnon favours devils over Christianity is because mortals seriously, seriously fucked up. It's been 2000 (less 600) years of divine wrath. Not a small mistake. Never should have gone anywhere near allowing Christianity to be possible, let alone actually adopting it.