There are various options. For as direct a comparison as possible, you need a Christian sect that is strongly influenced by Platonism and (Gnostic/Neoplatonist) mysticism, which then splits off from mainline Christianity to such an extent that it becomes its own thing. It doesn't need to be ethnically basis: the Druze weren't. They isolated themselves to keep their teachings "pure", and to keep them secret-- that last bit in part to avoid persecution.
The obvious call is Catharism, but I raised that suggestion in the thread about Cagots and surviving Catharism has been done before. So my suggestion here will be: some branch of radical Joachimites. Joachim if fiore got in trouble for his beliefs in OTL, and particularly because certain followers of his misinterpreted him and took too radical a stance. He had a distinct mysticist bent, which could easily inspire a group of followers who believed they possessed esoteric knowledge. OTL movements inspired by his views (most famously the Amalricians, the Dulcinians and the Brethren of the Free Spirit) were persecuted and destroyed in OTL, which would certainly prompt an ATL mysticist group of Neo-Joachimites to enact a policy of keeping their views very secret.
Other options include:
-- Some version/evolution/split-off of Paulicianism that goes big with the Gnostic influences and the esoteric mysticism.
-- Surviving Bogomils, likewise evolving into a secretive cult that closes itself off to avoid trouble.
-- Waldensians who don't get sucked into the broader Protestant movement, but spin off into their own distinct movement before the Reformation begins.
-- A late entry: some kind of proto-Reformationist movement with strong Platoniost influences that comes about due to the survival of Girolamo Savonarola. (The key difficulty here is that even if you remove the Borgias, who killed him in OTL, any other Pope will still consider him he pest who needs to be wiped away. The man was just making too much trouble to be tolerated. Yet you could imagine a movement based on his ideas surviving by fleeing Italy in some ATL, perhaps.)
-- A wild suggestion: the Rosicrucians (or some other band of Hemreticist mystics) make it big in the 17th century, and manage to become a tolerated cult in some part of Germany or something.
