Cults of the Ascended

Cults of the Ascended isn't exactly an organized religion - instead, it is more of a general brand for numerous lesser religions throughout the multiverse that are organized atop a relatively common theological basis. To speak of it in the simplest term, the Cults all follow gods that used to be humans (or, well, any other sapient), but managed to elevate themselves into godhood by their achievements.
The 'pure' Cults of the Ascended are best defined by what they aren't than by what they are. There are many demigods and gods serving the Church of the Highmost (although they are referred to as living saints and saints) or the Temple of the Seven (demigods and demi-archons) and so on. All of them become gods in Cults way, but do not consider themselves to be part of the 'Cults' themselves.
Thus cults are best described as 'independent' pantheons of System-born deities, without ties to any Power. While overwhelmingly powerful, Powers' perceive the flow of time in a different way, making them not be particularly interested in moving around quickly - the existence of such independent religions is tolerated (for the time being), as the Powers are mostly busy playing their own inconceivable games over the heads of the Gods (though with their position, they tend to be both pawns and players at once, to a degree).
However, the sheer number of the followers of the Cults is staggering - it's estimated that if you count out Ferra, close to 8 per 10 posthumans follows some of these Cults. Of course, none of the individual pantheons comes even remotely close to the power of its two main contenders, but as a whole, the Cults are numerous and well-entrenched in many civilizations throughout the multiverse.

Mythology & Lore

You can expect every cult of the Ascended to have at least a single mythological story, typically concerning their gods' rise to godhood and their prior exploits as demigods. The difference between that mythology and the 'classic' ones, is that this one tends to be solidly grounded in facts, although ones that typically happened in another world entirely.
Today the number of posthumans ascending to godhood has decreased notably - established pantheons do not like rivals, and unless you managed to become part of one, you will be a small fish in a very big pond. However this creates a situation where the mythology of some of the older gods tends to resemble 'classic' mythology - it describes things that are mostly forgotten (having happened up to five centuries ago, in a world where literacy isn't THAT common) or have been changed by the passage of time.

Granted Divine Powers

Naturally, gods can offer you some boons. However, unlike much more defined Church of the Highmost and Temple of the Seven, you can expect the available character classes and abilities to be shared by gods of the same archetype. So, for example, save for some exceptions, you can expect burial gods to have magic damaging to the undead, war gods to have similar combat empowering skills, etc. etc.
There might be some internal variations (for example a war god famous in his life for his spear skills will probably have combat empowering skills focused a bit more on spears), however the general similarities will be retained. There are cases of gods having access to more than one archetype, and there are some gods who are clearly an exception to all of that (however rare that is).