Yeah, I did this. Out of morbid curiosity converted most of India, parts of Persia, the Christian part of Spain, England and Ireland into what's essentially a "Christianity for the modern audiences". Attempts to convert the Byzantine Empire and France failed. The strategy is as follows:
- Go Scholar, Theologian perk tree. Amass piety and prestige by doing contracts, collect hooks on kings.
- Found a religion of your choice (or don't, if you're going with an existing one).
- Pick a base realm you're going to operate from (should contain a Holy Site of your faith).
- Convert ruler. If the holy site is under a lower level vassal, convert the whole ladder from the top down.
- Wait for the county to flip, this can take up to a couple decades. While waiting, attempt to convert any "low hanging fruit" available.
- When the holy site county flips, head there and begin your "Path of Ascension". First, consolidate your base by converting the entire realm to ensure that the king you converted earlier won't be replaced by a heathen heir. Then, expand - venture forth and preach
I managed to hit the gold when a Ghaznavid conqueror converted. He ruled most of today's Iran and Pakistan with an iron fist and was quite long-lived. It ensured that most of his vassals were forced to convert too, and it was kind of a positive feedback loop from there.
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