This could best be described as a "Ketamine Timeline"

Nothing so fantastical as that. There were sects of Christianity that followed more ancient traditions alongside their Christian beliefs. One popular set of knowledge (Christianity) and one arcane (Gnostic/Hermetic). You had to be initiated into the knowledge by a mixture of ancient and adapted rites. There were a number of disparate groups working from a similar or similar range of corpora.

The mystical timeline is parallel. There's a layer and then another layer. So what your common Catholic would have believed in 1500 was childish elementary mystical understanding to the arcane society member who knew the "real" or Gnostic truth.

I'm doing a terrible job at actually explaining this well. I'm giving you the very-diluted roundabout explanation of what I meant.

For a better understanding have a quick read through some Wikipedia articles. If your history of Christianity is not very thorough, then I recommend brushing up on that as well for a more rounded understanding. If you get into some of the reading, you'll understand 100% of those wacky conspiracy videos on YouTube and why people should read more, and how important context is.

My references are generally western-centric, though some of these topics make inroads to China and India (which has its own gnostic traditions IIRC).

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Down the rabbit hole you go:

https://en.wikipedia.org/wiki/Ancient_Egyptian_religion

https://en.wikipedia.org/wiki/Ancient_Semitic_religion

https://en.wikipedia.org/wiki/Hermes_Trismegistus (read many of the links)

https://en.wikipedia.org/wiki/Gnosticism (read many of the links)

https://en.wikipedia.org/wiki/Greco-Roman_mysteries

https://en.wikipedia.org/wiki/Theosophy

https://en.wikipedia.org/wiki/Aleister_Crowley

And down the Judeo-Abhrahamic-Christianity path:

https://en.wikipedia.org/wiki/Abrahamic_religions

https://en.wikipedia.org/wiki/Kabbalah

https://en.wikipedia.org/wiki/Christian_mysticism

https://en.wikipedia.org/wiki/Catharism

https://en.wikipedia.org/wiki/Sufism

and just keep going.

> My references are generally western-centric

that these strains predominantly dont exist in eastern christianity to me is indiciative that westerners were seeking to fill a spiritual void lost to 'their side of the fence' with other sources... as to the reasonings behind this, well, your 100% might come into play.

iirc Gnosticism and Manichaeism were both heavily influenced by eastern thought and took root in parts of the eastern Mediterranean all the way to Persia. The early church was very effective in labeling them as heresies and stamping them out.