Astute observation. I might add a nugget of interest: rationalism follows a rise in literacy rates. I don't know the tipping point, but as language fluency rises so do formal rationalist schools like Logic or Empiricism, mathematics, the sciences, you name it.
Your case is accurate, but I don't and won't follow with the rhetoric that it is deliberate degeneration. Humanists truly believe they are making moral progress towards utopia.
My point was that Christianity arose during a hot bed of religious sensibilities; none of which were divorced from their metaphysical sympatheties.
There was something rotten about the earth and the human condition that needed to be transcended and the deliverance was in heaven. We can see how later on, that fundamental view leads humanists to believe mankind needs to be remade, morally, and heaven ought to be on earth-in a sense.
Christianity really stressed the private life of individuals to such a degree that it shattered classical law which had no conception of the private individual. Jesus told his disciplines to work out their own sin and salvation in private. Later humanistic moralists like to point out the inconsistency between that and the institutional persecution of homosexuality. Humanists use the common trope that whatever one does in the privacy of their home without hurting anyone should be allowed.
Shit I'm ranting. Anyway I'm trying to connect the dots here without writing a book. It feels like We'd have to dig further into the past to look at the landscape before Christianity, and see if some proto humanism existed.
By stroke of luck, it could have been any one of the religious sensibilities of that time, Rome happened to pick up Christianity for reasons that are less than important to the topic.