The Ascending Path

The Ascending Path is one of the most ancient religious systems of Humanity - that, in fact, remained hidden for a majority of its history. It supposedly dates back to the ancient (and long forgotten) country of Erithia. Rather than organized religion it's something closer to pagan religions of old, many of which were actually at least partially influenced by hidden practitioners of The Ascending Path.
The Path itself is heavily disorganized, which makes trying to figure out its actual tenets something rather troublesome. It's partially religion, philosophy, and an eastern school of thoughts at the very same time. It's religious system is partially ancient paganism and partially a philosophical religious movement (like neoplatonism), but rarely one can encounter a more monotheistic strains of the Path.
Right now, however, only a relatively minimal amount of such movements actually survived. Overwhelming majority of current Ascending Path adherents in better times would be considered an abhorrent fiends. Practically all of better natured movements ended up long ago dissolved and unmade by the hundreds of years of Christian missionary work.

Mythology & Lore

The biggest problem of mythological part of The Ascending Path is that 99% (if not 100%) of writings aren't real. Books concerning the Path are mostly metaphorical, being pretty much glorified fanfics or ancient fantasy writings - but with often very real knowledge (about magic, for example) hidden deep within its words. Hidden or encrypted, and often both at once.
When a tome describes a myth regarding a Power doing something, it's almost sure that the author didn't mean for anybody to take that seriously (some people still do), but instead left scraps of his own occult knowledge in the text. Readable only by people of significant knowledge.
Completely separate branch of the Ascending Path mythology includes myths concerning past members of the Path (for example, tales of humans that later on ended up becoming Ascendants (probably), and histories of former cults dedicated to following the Path.

Divine Origins

The Ascending Path originates in the dark age before the writing was first developed. Some claim that it's as old as agriculture, and that it was created in the lost land of Erithia, somewhere in the Middle East region. Few scholars even suggested that it might be the oldest religion of Mankind, from which all other developed.
Frankly, finding out details is practically impossible right from the start.
Thousands of years passed. Hundreds of writers wrote (or sculpted) sometimes fake histories of magical antiquity. At least three different groups - Unseen Legion of the Roman Empie, Islamic Caliphate under the rule of an insane Caliph Al-Hakim bi-Amr Allah, and Catholic Church - also made extensive attempts to wipe out all traces of Erithia and the origins of the Ascending Path from the face of the world.

Cosmological Views

Divine Hierarchy

Four Primeval Powers

Before time existed (and before the world as we know it was), there was nothing - with exception of Four Primaeval Powers. Those basic 'entities' (that actually are seen as non-sapient powers by many if not most of the schools of thought remaining within the Path) were: Creation, Stasis, Change and Destruction.
That's where the various schools within the Path varies. Some see the Four Powers as elements of a central, deistic Creator-God (that, in time, will be a Destroyer-God). Others see Creation and Change as a singular power, with positive connotations and claim that Stasis and Destruction oppose them. Some schools replace Stasis and Change, claiming that stability is better than chaos, while others claim them to be a singular being of balance between Creation and Destruction.

Ascendants

Nature

Beneath the Four Primeval Powers (or their equivalent in that particular school) are the Ascendants.
The whole point of the Path is to either achieve ascendance into a de facto godhood - or die trying. Literally. Of course, a person that dies without achieving ascension (99,9999% of adherents end up like that) aren't simply left alone. They follow 'their' Ascendants into an afterlife. If they were faithful, of course.
The problem with the Ascendants is that their exact nature is dubious. Nobody truly knows details about mortal lifes of Ascendants prior to achieving the peak of what human can achieve while walking the Path. There are myths, but... who knows the truth? There are also many cults whose approach to them is pretty similar to the one that pagans had (or have, outside of Europe) to their gods.
Many Ascendants are actually suggested to be a source of inspiration for said pagan gods. While sometimes being suggest to BE said pagan gods. Situation varies from book to book and cult to cult.
Some cults divide them into two main groups. Greater Ascendants includes six Ascendants believed to be strongest of them all (at least by a small majority of cults). Their existence is often criticized as an aftermath of manichean/zoroastrian influences dating back to the times where the cornerstone of the Erithians power lied in the Persian Empire and Eastern Roman Empire.
Others claim that it was the dualistic Ascending Path that led to creating of manichean and zoroastrian religions.
They Greater Ascendants are directly connected to the Four Primeval Powers, and because of that the belief in their uniqueness is strongest in the cults that see the Powers as active powers, antagonistic to each other. Even the cults that do not see accept the division between Greater and Lesser Ascendants generally see the six Ascendants to be the strongest (or at least most prominent) ones of them all.

Principles

Ascendants (and entire thaumaturgy) are divided between Principles, basic 'powers' and aspects of magic (each of them said to hide a path into Ascendence) that they personified. Magic users that are not being a followers of the Ascending Path still use the Principle system for their magic, even if they generally disregard Ascendants and instead follow its 'pure' form. Said Principles are as follow:
Silence - Principle of weakening, eradication of life and death. The ancient 'white' Ascending Path adherents saw it as a part of a rites of passing and Silence being a source of ultimate relief and eternal sleep. The Forbidden see the Silence as source of powerful curses and necromancy. It is said that following Ascendants of Silence allows one to achieve immortality via becoming undead.
Heart - Principle of lifeforce, life and sensations. Just as in case of the Principle of Silence, Heart interpretation varies heavily between sects. While magicians once seen it as as a source of life, birth and health, this interpretation is mostly dead, replaced with Holy Magic. The Forbidden see it a source of pleasures and self-indulgences, with the ammount of sexual perversions and sex cults being especially high amongst followers of Heart. It is suggested that the best followers refine their bodies to be eternal.
Anvil - Principle of remaking, modifying and improving. One of the very few things that both variants of Erithians would generally agree upon. A source of both techmaturgy and at least some buffing magic. Seems to be growing in power together with mundane civilization. According to specialists, following Anvil leads to an increased degree of mechanization that leads to immortality.
Gold - Principle of intellect, truth and so on. Useful for both detecting traps and improving your mind. Perverted by the Forbidden into either merciless nietschean approach to self-improvement or using it to enforce their false truths. Those that follow it with extreme devotion and sharp intellect are said to be able to ascend into a form of an energy being.
Mirror - Principle of lies, illusion and deception. Ancient Erithians saw it as an antagonist of the Principle of Gold. Today, however, the Forbidden see it as its brother, as all truth must be guarded well, while an intellect could be used to see through lies. Dedicated followers of Mirror are said to ascend into a place behind the mirrors... unless it's a lie.
Sword - First of two Principles connected to combat. Sword is a Principle of attack, conquest and war. Those that dedicate themselves to this Principle can theoretically achieve a transcendent level of combat prowess, becoming something unstoppable and unkillable. Avoiding even natural deaths.
Shield - Second of two Principles connected to combat. Shield is a Principle of defence (both personal and strategic), and war. Those that dedicate themselves to this Principle can theoretically achieve a transcendent level of combat prowess, becoming something unstoppable and unkillable. Avoiding even natural deaths.
Crown - Principle of control, over both minds, animals and elements - the understanding of that principle is based solely on the magician. Needless to say, control of the elements isn't really enough for the Forbidden. Most of them look to control everything. And everyone. The actual peak of the Principle of Crown is said to be an an evolution into a purely mental being, capable of inhabiting bodies at will - and controlling others. Or, becoming a being of elements.
Key - Principle of opening things. It can include both locks and the hidden doors of Reality, allowing the caster to summon things and gain knowledge that he shouldn't really have. Even altering reality. The specialists of the Principle of Key can become one of the things dwelling outside.

Tenets of Faith

Different for every of the Ascendants - as every of them has different tenets of faith, slightly different ethics (either very grim or total lack of them in case of Forbidden), different priesthoods etc. Instead of trying to describe it that way, it will be better to focus on Ascendants in whose footsteps they walk on the Path.

List

Greater Ascendants(?)

This part includes small group of beings that are considered to be the most powerful of them all, sometimes seen as prime servants of the Primeval Powers. Each of them probably played at least some supporting role in history, even of the mundane world.
Sun Everlasting - Ascendant that embodies sun, light and (relatively) benevolent rulership. The last one changed a lot towards tyranny after the Forbidden took over. Almost raised to prominence in the late Roman Empire, where it carried the name Sol Invictus. Lost to Christianity, and now the cult changed.
The Sun Everlasting is the prime executor for the Powers of Creation and Stasis (used to be Creation and Change, but among the Forbidden the alignment changed). Pretty detached from the rest, just as his counterpart, the Night Eternal. Some myths claim that they are parts of a single being, perhaps some deistic creator being sometimes called Demiurge. Many sources indicate that they might not be in fact Ascendants, some even go as far as to consider it a source of the concept of a godlike being in three persons).
Great Mother - Ascendant that embodies sensations, pleasure and birth. It was mostly the last one among the Whites prior to the Christianity taking over, now the former two seems to be much more important. Serving her does grant new, and unique sensations... though this also includes this rather unpleasant. Her Principle is Heart, and she serves Creation. One of the most ancient of Ascendants, often associated with at least several pagan deities (including Astarte, Cybele, Isis and Ishtar).
Crown-Of-Ages - An Ascendant of embodied stasis. A patron of stable, hereditary society. Used to justify both the existence of centralized government (or, in fact, any sort of it other than total anarchy) and slavery alike. Whether he was seen as connected to any particular gods of power and control is unknown (but many suspect that deities like Moloch and Baal were seen as its incarnation. His Principle is Crown (not very surprising), and he serves the Primeval Power of Stasis.
Night Eternal - An Ascendant that oposes the Sun Everlasting. Had several names, mostly as gods embodying anarchy, death and destruction. Persians knew her as Ahriman (though her gender is up to debate at least partially). Violently opposed as deity of evil by the Whites prior to conversion, but the black outsiders see her as a creature of freedom, ambition, strive for power and black magic. Serves Destruction and Change.
Just as Sun Everlasting, she is rather withdrawn, which paves a way for her sidekicks. Some myths claim that they are parts of a single being, perhaps some deistic creator being sometimes called Demiurge. Many sources indicate that they might not be in fact Ascendants, some even go as far as to consider it a source of the concept of a godlike being in three persons).
Worm-That-Walks - Ascendant that embodies death and decay - that pave the way to eventual rebirth. Or, at least, that's how the adepts of old saw him. The black ones now see him more like a patron of filth, decay and death in a rather straightforward way. He is also called Herald of Filth. His Principles are Silence and, surprisingly, Heart (since worms are a type of life as well). His symbol is a coiled worm. Some say that all gods of death and underworld were Worm-That-Walks, but equally many claims otherwise. Serves Destruction, obviously.
He Who Lives Behind - A creature of personified change, deception and lies. An Ascendant of a thousand faces. Erithians before converting to Christianity saw him as a patron of arts, today it's just deception, illusion and lies. Its Principle is Mirror, and it serves Change. A thousand theory of what the name means, none of which probably makes sense. It's said that it was venerated as many deities, all of them equally fake.

Lesser Ascendants

The Patient Hunter - Ascendant that embodies strife, self-improvement and war. His Principles are Sword and Shield. According to the Forbidden, a hunter that searches for strong prey, a general that seeks to conquer, a warrior that trains and fights - it's all his domain. Needless to say, Ludendorff must like him. He is also an extreme social darwinist and believer in the idea that you should be able to kill whoever's weaker than you and take everything they had for yourself.
Rapturous Storm - Ascendant that embodies the lack of restraint in pursue of ultimate ecstasy. A personification of hedonism beyond the point of any sense and beyond the rational thought. The peak of his power among the mundane world was cut short when the Roman Republic brutally suppressed the Bacchanalia. He achieved some level of resurgence amongst the Forbidden. His Principles are Heart and Key.
Perpetual Contraption - Ascendant that serves as a patron of machinery and techmaturgy. It is in his name that the techmaturgists of the Greater German Republic construct new machines of destruction and terror. Said to be a machine more than an Ascendant in traditional sense of the word, some also say that the Antikythera mechanism was one of the tools he used for his Ascension. His/It's Principles are Anvil and Gold.
Illumination - Ascendant that embodies mental improvement and knowledge about the way the world works. In short - physics and other, basic knowledge. If he wasn't explicitly against 'lies' like mercy or researching for common good (only about the knowledge on its own, done for itself. FOR SCIENCE, even if you need to kill people for it), he might have find some fanbase beside the crazies. But, well, nope. His symbol is a burning candle. His Principle is Gold.
Reflection - Ascendant heavily connected with mirrors, governing things that can be summoned from them (or by using them). Extremely mysterious, some ancient writings indicate he doesn't even exist, being merely a reflection of 'something' living in the mirrors. His Principles are Key and, not very surprising, Mirror.
Watcher-At-The-Edge - Ascendant that is said to be a personified Principle of Key (though he is often described as both key, lock and the gate as a whole). He observes everything from beyond of what we consider Reality, overseeing (and BEING) gateways to places beyond. He can open these gateways... for a price, naturally. Even more mysterious that average Ascendant.
Moonlit Anvil - Ascendant that is said to be the source of legends about werewolfs (or, true werewolves, if they were around, sources vary). He is a servant of Change, both true and illusive one (hence his Principles: Anvil and Mirror). She is believed to be connected to many lunar deities in the history. Many abominations were made in her name, and many people lost their humanity.
Broken King - Ascendant that is said to be dead. Nobody actually knows when or how it happened, but most sources agree on that part. There is still some faint live in him, enough to has some influence in the world today (or maybe his death means something else than it does normally). Even the Forbidden are wary of unspeakable cruelty and singular hatred to the world that the King once had. His love for blood offerings, torture and extinguishing lives is endless. His Principles are Crown and Silence.
Void - Ascendant that is said to have achieved a state in which he both exists and doesn't exist. He embodies refusion and negation. Some specialists suggests that he was beyond the creation of buddhism - the idea of refusing desires to achieve inner peace sounds certainly similar. He seems to have fell into some sort of madness in the meantime, as now it's a being of a pure rejection and negation of everything - if Void was allowed to spread its influence without limits, he would unmake entire Creation. His Principle is Silence.