There is a profound distinction to be made here. We could both, Custodian and Grey Knight, slay daemons. We were both, to all intents and purposes, immune to their temptations and we were both effective against their many stratagems. There are two great repositories of lore against the daemonic in the Sol system: our own archives in the Tower of Hegemon, and the far greater librarium lodged on Titan itself. We are, as orders, steeped to our very cores in the fight against the Great Enemy. Perhaps you might say, Chaos is the reason for both of our existences.
And yet, we are different. Remember I told you that we were never warriors, not exclusively. We are certainly not an army, and we were intended, in the original scheme, for service in an empire that never came to be. We always knew of their existence. There are records, held privately in the depths of our archives, which chronicle their creation. We watched, ten thousand years ago, as He embarked on His last gambit. As the Great Enemy drew close to Terra, we observed the darkening of Saturn’s moon, and knew that one day it would return, its purpose fulfilled.
Consider what this history means. We know that they came after us, the more junior creation, and yet they were as closely associated with Him as we were. We both of us look to Him and Him alone as our progenitor, and share the same sense, cultivated over the wearing aeons, that we enact His designs when all others falter.
There are some among my brothers who do not see the sons of Titan as much more than specialised Space Marines, to be regarded with suspicion as part of that schismatic breed that caused us so much anguish in the past. A Space Marine may always fail, they believe, given enough time and enough reason, and thus they are all part of the same potentially aberrant strain.
Some think that. Others, and I myself have often speculated in such a vein, cultivate a different misgiving. We know well enough that they were designed as His last great weapon, fitted to an age that He foresaw near the end of His earthly embodiment. What if it were they, not us, who most faithfully embodied His final legacy? You will never hear one of us say as much out loud, but that does not mean the suspicion does not exist. It skulks around the corridors of Hegemon like a foul odour, faint but hard to eradicate.
From the speculum certus [the public and secret records of the Emperor's orders and speech] we know we were the finest and the most faithful. In the speculum obscurus [things they infer from the Emperor or believe he would have told them in time] there is, as always, more doubt.