The One and the Nine
Notes on the Chapter Cult and its influence on the local Imperial Faith

Opening Thoughts

While I do love to read sourcebook lore and do occasional deep dives on stuff that interests me, I started building my chapter more than 15 years ago with little knowledge of the universe beyond what was in the Assault on Black Reach Booklets, and even as my knowledge expanded, the rule of cool was my primary rule and GW was offering wicked cool kits with great looking bits for a space wolves, blood angels,  grey knights and others. After I encountered the b&c (many years ago now) I have been off and on working my Index Astartes for many many years now, and trying to figure out an in universe justification for why I have more out of chapter bling than the Blood Ravens ;). Eventually I tied it into the chapter cult, as there is no  better answer for cultural assimilation and  clandestine artifact acquisition than Religion, as history has proven to us so often. Anyways, here is the outline for the chapter cult and organization of the chaplaincy.

Key Features

The Iron Boots worship the emperor as the god of humanity, viewing him as more than just a man, as opposed to the majority of Astartes chapters that view him as just a man. His 9 sons, progenitors of all loyal Astartes chapters, are worshipped alongside him as demi gods. Within the Iron Boots, there are orders dedicated to each of the 9 primarchs, some of whose members have become so devout they develop preferences and stigmata based on their patron primarch. Brothers are not required to join an order, but the majority of the chapter generally does choose one primarch to follow at some point in their life.

The Iron Boots are dedicated evangelists, spreading their cult to the worlds they have retaken in the Emperor's name, with the local branches of the Imperial Faith for the most part having taken on the same or similar doctrines as the cult.

The individual orders within the chapter have been known to send search and rescue and  archaeological expeditions to warzones and famous battle sites where descendants of their favoured primarch have fought. They do this  both to assist those they consider kin and to retrieve lost artifacts and materiel of their patron primarch, should the opportunity arise.

Brothers belonging to an order will often show this in their personal heraldry and trophies that they carry into to battle with them. If a full squad happen to all be dedicated to the same order (generally the case only with veterans of the first company, to help prevent internal rivalries becoming anything more than friendly) they may choose a unified way to present this, such as a custom paint scheme for the squad.

Organization of the Chaplaincy

As could be expected from a chapter that places such emphasis on faith and the divinity of the emperor and his favored sons, the Iron Boots boast a larger than normal Chaplaincy, with multiple chaplains assigned to each company (typically 2-3, but as many as 5 at certain points in the chapter's history), generally more than enough to have one accompany almost any mission, even minor deployments. While it has been known to happen that a chaplain will choose no single preference, the great majority of chaplains choose a patron son of the emperor to venerate, though they will of course provide counsel to any battle brother that requires it.

Heading the chaplaincy is the Chosen of the Emperor, generally the most senior chaplain in the Chapter, but as they are nominated by the Council of Elders, a different member is sometimes selected. The Council of Elders consists of the Reclusiarch that is appointed for each chapter cult (typically the most senior living member) and any chaplains with a hundred or more years experience, known as Elders.  Rounding out the ranks are those Chaplains with less than 100 years of experience and finally, Acolytes, brothers still in training to become a Chaplain.

A Mission, consisting of an Elder and a small group of Chaplains and Acolytes, is often assigned to a newly conquered/purged worlds to assist the Ecclesiarchy with spreading the Imperial Truth, in particular the version espoused by the Chapter itself.