This is expanding on my matoran pantheon.
Throughout the island of Okoto, there are various religious organizations devoted to the many gods and spirits of Okoto.
The Crafters of Artakha: The crafters of Artakha are a group of skilled craftsmen from all over the island to believe the path to the land in the skies, Artakha, is through crafting great works. They are known for their mask creations as well as many others. They worship a number of gods, mainly Artakha, Ekimu, and sometimes Makuta. They have stopped worshiping Makuta for the most part due to the bad image the brotherhood gives him.
The Brotherhood of Makuta: The Brotherhood of Makuta is an extremist militant cult devoted to Makuta. They believe that he created the rahi as the intended inhabitants of Okoto and that they are better than matoran themselves. As a result, many of them have performed experiments on themselves to fuse with rahi, becoming known as Rahkshi. They have also summoned various evil spirits from the realm of Karzahni(the kraata powers) and used them to become Rahkshi as well. They eventually plan on releasing the Bohrok, a race of condemned matoran that live in Karzahni, upon the world so that they can cleanse the island of matoran so the rahi and rahkshi can live in peace. Due to their spread of fear, they have caused many other religious organizations to revile Makuta so as not to be associated with the brotherhood.
The Kohlii League: Who would have thought Kohlii would be a religious organization? Well guess what, it is. Ketar, the patron god of Kohlii and spirit of stone is worshiped by Kohlii players in hope of doing better. Idols of him are made at Kohlii stadiums and ritual feasts are done before Kohlii matches in order to thank Ketar. If these feasts are not done, it is said to incur the wrath of Ketar.
The Sibils of Umarak: Umarak, the swamp spirit, is said to be a messenger between Akida and the mainland gods of Okoto. He is said to bring messages through swamp gas, a highly toxic and volatile natural occurrence that causes hallucinations and plays with light. The sibils of Umarak are a group of (extremely) eccentric matoran(primarily Onu-Matoran) that breathe in the swamp gas in an attempt to read the messages and learn about the future of Okoto. This causes them to speak gibberish and run around in circles. These frantic movements are translated by other matoran who contrive from them the future of Okoto. The predictions are either spot on or completely off. Very desperate matoran come to the sibils but barely anybody else does. In a recent prophecy, a matoran contrived that the island of Okoto would be cleansed by Bohrok, damned matoran from Karzahni, and anyone who helped would be rewarded. As such they have begun to work with the Brotherhood of Makuta in unleashing the Bohrok. Because of this, Umarak's already foul name(he's the spirit of ogre infested disease holes/swamps) has become even more foul.
Toaism: Toaism is the dedication, study, and worship of one of the mythical Toa in clandestine monasteries across Okoto.
The Book of Virtues: The Book of Virtues is a staple to most religious groups and isn't actually a book most of the time. It is usually told orally by a Turaga scholar. It has three books within it. The books aren't really in chronological order and are really just the compiling of several different religious texts.
The Book of Unity: The Book of Unity describes the creation of the world and mainly details the life of Ekimu, God of Kindness and Unity. It also talks in detail of the land of Artakha.
The Book of Duty: The Book of Duty lays down many laws on how matoran should work and treat each other. It talks of the land of Karzahni where bad matoran are sent and how the matoran become Bohrok, beings designed to cleanse the island of Okoto the moment it was deemed un-pure. That particular section is where most of the stuff the Brotherhood uses comes from. It also talks about Makuta, God of Duty and Justice.
The Book of Destiny: The Book of Destiny details the history of the matoran and various predictions that will happen in the night sky. It also talks of Mata Nui, God of Destiny and Prophecy.
The Book of Elements: A compiling of the different stories about the different elemental gods.
The Book of Toa: The same thing but with Toa. Each Toa gets his/her own chapter. Toa are said to be mythical warriors who climbed Okoto's highest mountain and were deemed worthy by Mata Nui to protect Okoto. There are thousands of different Toa worshiped by different Toaist Monasteries.