I remembered I made this in the past:
Let's compile what we know of Trelagism and Trellism.
1) Trelagism was a religion on Scadrial during its Classical era, before the Lord Ruler's Ascension. It was practiced by the Nelazan who lived close to the poles. They mapped out the various astronomical bodies visible from Scadrial.
2) They believed that Trell and Nalt were brothers. The latter was jealous of the former and cast his single burning eye (the sun) upon the lands to block out the Thousand Eyes of Trell (the stars)
1) Trellism is the belief in Trell. Is associated with a new metal which is described as silvery with distinctive, dark red spots. It has only been seen used Hemalurgically so far: on Kandra and to create Chimeras
2) Several members of the Set, in both high and low positions, are followers of Trellism, and the organization itself may exist for the purpose of serving Trell. Bleeder was also turned by Trell.
3) Trell has their own Faceless Immortals, who are described as having red eyes. A red miasma is how Harmony chose to represent Trell to Wax in BoM. They are also associated with "men of gold and red, bearers of the final metal"
4) There seems to be a strong element of anti-establishmentarianism in Trellism, namely: freedom from Harmony and social justice from the central government in Elendel or more broadly the Basin
Now the most popular theory (and the one that I subscribe to)
Trell is Autonomy or one of her Avatars
1) The stated motives of Trell's followers align well with Autonomy's Intent
2) White Sand spoilers
3) Trelagism's reverence for stars aligns well with Autonomy. She seems to currently be inhabiting Taldain's star.
4) The long day/night cycles experienced by the Nelazan is reminiscent of Darkside Dayside division of Taldain
5) Autonomy is said to have multiple Avatars. There is a thematic resonance with worshiping a multitude of astronomical bodies
6) Autonomy's star might be visible from Scadrial.
7) Trell is a Shard we know. It isn't Endowment. Honor, Ambition, Devotion and Dominion are Splintered. This only leaves Cultivation, Odium and Autonomy. The former two are embroiled in the conflict on Roshar.
8) Autonomy is stated to have a habit of interfering on other worlds by Khriss
Trell is Odium
1) Has a track record of attacking other Shards. Odium is known to fear Harmony.
2) The break between Stormlight front 5 and back 5, during which the Wax and Wayne era books take place
3) Is known to corrupt Investiture (red)
4) Similarities with religions on Roshar
5) the agents of Trell align with Odium's Intent.
6) Alongside purple and black, gold is a colour associated with Odium. "Men of gold" might also refer to
7) Odium's forces comprise of several beings that may pose as Trell's Faceless Immortals:
8) As per WoB, Odium has visited Scadrial before.
-1) Raysium does not look like Trellium
-2) Odium is restricted to Roshar. (Note that Taravangian said there were holes in the contract plus Odium might've employed some other methods like sending Splinters or Avatars)
Both Autonomy and Odium
1) May have worked together in the past to Splinter Honor, and before that Devotion and Dominion, the former sounds like an opposite to Odium, the latter to Autonomy.
2) possible red herring in a broadsheet in era 2 "Vote for Passion, Vote for Freedom"
3) the break between Stormlight front 5 and back 5