I'll put a disclaimer here that it's been a long time since I've interacted with BBcode, so I'll make everything pretty after I put some stuff down.
"...And in many ways, ours is a strange and liquid creed, like ferrous itself, when put to heat and pressure. Yet it is by that very pressure that iron is melted down and shaped into something harder, more lasting, and tempered into whatever tool we are molding. Make no mistake, our creed is a tool, and to wield it we must understand it's purpose in the Imperium, and with that understanding we clarify our own. Tolerance, brotherhood, acceptance - these are what we must exemplify and offer to all of ours who fly His holy Aquila, while chastising and rejecting all such things to those who have, in turn, rejected the Imperium.
On the surface, this might not seem such a difficult thing. But should you live long enough you may very well see those you embraced, broke bread with, fought and sacrificed for turn away from the purpose you once shared. You yourself may even do so, even though such a terrible thought may seem the greatest of heresies to you now. Even the Arch-Heretics were once the Emperor's Children. Even they, were once our brothers. And it is the repeat of that betrayal that we must guard most fervently against." - High Chaplain Varstead of the Echoes of Iron.
It was in the Third Founding that the Echoes of Iron were formed, from the struggling remains of Ferrus Mannus's geneseed. The first Chapter Master was an old veteran named Tor Artellius, a witness to the Horus Heresy when he was young. Chapter Master Tor was chosen because, in addition to his vast experience, he seemed most determined to return the Iron Hands to the former power and glory of the Legion of old, to rebuild to the place where the Chapter he was now responsible for would contribute wholly again to the Great Crusade of reclaiming the galaxy for the Imperium of Man.
And this the Echoes seemed determined to do. They took up their mantle with zeal and passion, quickly establishing a Fortress Monastery on a stable world named Recusatio X. First Captain Elethuar Vardois establish a rigorous and scalable training process for recruiting youths into the Chapter, having veteran Guardsmen training boys in a mirror formation of the Codex Astartes to prepare them for the duty they would have to face. Chief Apothecary Cordwain and Brother-Magos Erstwin established procedures to screen recruits for their chances of rejecting the geneseed, and to reward the families who produced sons that accepted the geneseed with greater ease. In turn for the Astartes protection of Recusatio X, the planetary governors would protect and elevate the bloodlines whose scions most easily accepted the geneseed of Ferrus Mannus, ensuring that those bloodlines would reproduce, and testing the rest of the population when census was taken for other potential candidates.
This involved process ensured that in the early days of the Echoes of Iron, the Chapter's growth was both stable and quick. After a few generations of planetary governors honoring the process with less and less oversight by the Chapter itself, the Echoes trusted in the tradition they had established, and largely turned their attention to internal affairs and using their burgeoning strength for the glory of the Emperor and His Imperium. The specific turning point of this was reached when news of new Ork Waaagh! was threatening the nearby star sectors, and the Echoes turned to mobilize in force against it.
For a century and a half the Echoes first countered, then crushed, then finally sought to chase down and cleanse the Ork menace that had risen up. A full fifty years was spent in flux between stationing entire Companies to defend the threatened worlds and deploying small squads of First Company veterans to assassinate Orkish leaders to try to destablize the Waaagh! and cost it momentum. As the tide shifted against the greenskin menace, the Echoes rallied the Guardsmen and Naval allies to take the offensive against the Orks, who remained intentionally mired in civil war and infighting, until at last the Waaagh! broke. From there, the Echoes mobilized to follow the Orks back to their source, and burn out whatever hole they spawned from, and claim what resources remained for the Imperium.
It was in this crusade that the ever subversive forces of Chaos saw their opportunity. Worshippers of the Prince of Pain and the Skull Throne, scattering from where the Hammer of the Emperor fell on them elsewhere, quietly slipped into the populace of Recusatio X. They found themselves a place among the underworld of that society, of those already dodging the census and the testing that it brought. Mothers who did not want to lose their sons to Astartes recruitment, mutants that did not want to be discovered and cleansed, and dozens of other stripes of criminals who had established a second society that quietly evaded the good Imperial powers of Recusatio X. In this haven for those evading Imperial purpose, did these cultists find a verdant soil for the seeds of Chaos.
I am not finished, of course, but I wanted to put something down before I got distracted again. I really don't know how I feel about the last paragraph, and am inclined to rewrite it entirely. For now, however, what is there is free to critique.