These are just a series of semi-coherent ideas about the Matoran religion (which we'll call Nuism for brevity's sake). Just bear with me. No, I will not try to write this as Vakama would tell it. I'm too lazy for that. Note that this is only the religion held by the Matoran of the main 6 villages; other regions, which would be added on later, might have differing beliefs.
-The creation myth-
Mata Nui, decided one day that he would make stuff. And lo, there was stuff. He began by making the world; then he formed specific regions from which the elements would be derived. To populate these regions, he made the Matoran, his chosen people, and relegated specific races among them to inhabit specific regions. To manage the universe, Mata Nui created the rest of the Great Beings. One of them, the Makuta, betrayed Mata Nui (his motivations for this would be explored in later parts of the sage) and plunged the world into darkness with his Rahkshi. To combat this, Mata Nui set up a process by which worthy Matoran could become Toa, whose powers are manifestations of the regions they come from, to defeat the Rahkshi. The Toa were victorious, and Makuta was banished to the underworld. To the present day, Toa have continued to be chosen to defend the world from the Makuta's salt and outdated memes. And that's where babies come from.
-The Great Beings-
The Matoran consider 6 to be a holy number. As such, they have 6 deities whom they worship. This excludes Mata Nui himself, who is regarded on a higher level. These spirits do not meddle in the affairs of the material world very much, save for when the Matoran seek their guidance or help.
1. Teridax, the spirit of thought, the mind, and politics. He is associated with peace, and with Ko-Koro.
2. Artakha, the ruler of the Red Star, where virtuous Matoran go when they die; and the god of wisdom and knowledge. He also serves as the patron god of scientists and inventors. He is associated with prosperity, and with Onu-Koro.
3. Karzahni, king of the underworld and tormentor of sinners. He transforms the bodies of his victims into mechanical slaves called Bohrok, which are controlled by his 2 pet dinosaurs, Cahdok and Gahdok. Karzahni is also a war deity. He is associated with courage, and with Ta-Koro.
4. Ekimu, the patron of the arts and mask-makers, who hold great importance in Matoran society. He is also a protector of households. He is associated with creation, and with Po-Koro.
5. Vahi, who presides over the passage of time and holds together the fabric of the universe. She is associated with purity, and with Ga-Koro.
6. Ignika, spirit of life and death. He acts as a psychopomp, and rides around on a flying surfboard. The existence of Rahi is attributed to him. He is associated with faith, and with Le-Koro.
The Makuta is excluded from these ranks because of the aforementioned Satan-like role he plays. His betrayal is also the reason why a 7th wheel in a group of 6 is often looked upon with suspicion, for this is seen as a bad omen.
In addition to these, the Matoran believe in various other spirits which preside over the elements. They are not worshiped, but may be prayed to in order to earn their favor or protection. These include Ikir, Uxar, etc.; these creatures' Rahi-like natures are what separates them from the true gods. The species of Rahi which they most resemble are also considered sacred for their association with these "primordial creatures."
-Ritual and places of worship-
The Matoran do not have dedicated churches. Prayer is often conducted in the home or place of work whenever it is considered necessary, particularly on holidays. Matoran generally keep a small shrine to specific gods (usually Mata Nui) in their house. There are, however, temples where Matoran may go to seek spiritual guidance.
Of these, the most noteworthy is Kini-Nui. Stationed just outside of Ga-Koro, it is a grand temple where a society of Matoran monks live and curate the building. This is where pretty much all Av-Matoran live. It's located on a mountainous island where clouds rarely gather, thus letting light shine upon it most of the time; and even at night, it is bathed in light from the many giant lightstones stationed around the temple. When clouds gather overhead, it is seen as a sign that evil is coming. Kini-Nui is also the site of the Toa Suva, where Matoran take Toa stones to become Toa.
-Toa and their significance-
In the Abrahamic faiths, there are saints. In Nuism, there are Toa. Becoming a Toa is a great honor, as Toa are seen as the chosen champions of Mata Nui himself. At Kini-Nui, the monks watch out for meteors containing Toa stones to fall to the planet's surface. Once they arrive at the landing site of the meteors, they take the stones in the direction where they shine brightest when aimed, and are given to whatever Matoran causes them to glow the brightest. The Matoran then take the stone to the Toa Suva, where they become Toa.
Of all Toa, Toa of Light are considered the most powerful and regarded the most highly, as the element of Light is thought to be the first that was created. Shadow, the last and most reviled element, is not used by any Toa or latent in any Matoran (unless Umarak is implemented as an antagonist). Toa of Light are rarely chosen, but when they are, they come from Kini-Nui.
-Societal roles-
The Matoran only have one spiritual leader in each village: the Turaga. These are best described as priest-chieftains, and preside over the Matoran theocracies. Kini-Nui does not have one, but the other villages all do. The duty of a Turaga is to lead their people's worldly affairs and meet their spiritual needs. If they fail to do so, the people are capable of overthrowing them. When the incumbent Turaga dies, all candidates go to Kini-Nui, where a ritual is held that decides which one will be the new Turaga.
Of the Av-Matoran, there is also a Chronicler, who goes about the world recording history and current events. The importance given to stories and narrative makes this a sacred position to hold.
-Philosophy and Morality-
The Matoran believe that virtue comes from following the sacred Three Virtues bestowed upon them by Mata Nui. These, they believe, are what make them higher than mere Rahi. Unity, Duty, and Destiny must all be abided by to gain entrance to the halls of Artakha. Causing disunion among oneself or others, not following one's duty, or not taking control of one's destiny will land sinners in Karzahni, otherwise known as "the suckhouse" because it sucks. The Three Virtues' relevant symbol is the official symbol of Nuism.
The concepts of peace, prosperity, etc. are all thought to be different attributes linked with certain gods, as listed above. These are also considered vital to virtue and goodness.
Great importance is given to storytelling, and the narrative of the universe and the plight of good vs. evil (called the "Bionicle") is considered most sacred.
Alright, that's all I have in me. Just some thoughts of mine that I farted out for whatever reason.