About Stendarr's Orders - Lore and background of Stendarr's Paladins

So, recently I have this ''obsession'' with all things Stendarr. Especially his esoteric, fanatic religious and knightly orders like Knights of Stendarr and Vigil of Stendarr. I'm interested in lore behind the Vigilants, if there is any outside of what we learn in Skyrim, which is very little.

I've heard of Resolutes of Stendarr, but am very interested in hearing who and what were they exactly. In the books The Four Abominations and Stendarr's Divine Spear I recognize ideas and teachings shared much later by the Vigilants. Archbishop Vinicius Imbrex is the author of ''The Four Abominations' and is from Chorrol, known as the center of Stendarr's religion. But this was written all the way back in the 11th century of the First Era. This means that Vigil's ideology is almost ancient, and not a result of Oblivion Crisis. The Crisis could just reawake those old Stendarr's paladin orders in the form of the Vigilants. So, they are basically a continuation of an old anti-daedra tradition. Is it safe to assume that Chorrol still serves as a center of Stendarr's paladins?

I guess, my another question would be - why Stendarr? His religion seems almost more esoteric than that of Akatosh, certainly most secretive and serious out of all Aedric cults. Perhaps i'm wrong. But we know that Knights of Stendarr existed entirely separated from the Knights of the Nine. A quote from wikia:

The Knights (of Stendarr) have been seen fighting alongside the Knights of the Nine against the Daedric attacks in the 4th era. The Knights of Stendarr have also been known to protect cities in Cyrodiil and Hammerfell. They are widely spread through Tamriel and some of their land is still undiscovered by people outside of the order.

It's not just that they fought alongside Knights of the Nine, but their land is undiscovered by the outsiders. What's the reason of the secrecy and esotericism shared by all Stendarr's paladins in all shapes and forms? Is Stendarr's magic and influence legit in TES universe or is it just a propaganda?