...a letter with the unfakable seal of the Ordo Hereticus on it...
...That is a seal that cannot be faked...
Anything which can be created can be duplicated, especially if people are wandering around the galaxy carrying copies with them. Even more so when the forgery need only pass a cursory visual inspection.
Also, Space Marines respect the chain of command, when a letter comes from Holy Mother Terra, they tend to not question it. If there is one thing you need to learn is that Humanity is kinda stupid... Just, as a rule in 40k. I mean, their whole system is based on the ideology that "To question your orders is to know doubt. To know doubt is heresy, which is punishable by instant execution."
The Atartes follow the Emperor, not Terra, and usually in their own way. Since the First Founding all Space Marine Chapters have been autonomous and function under their own creeds and command structures, that's one of the reasons they manage to have things like their own religious practices without the Ecclesiarchy coming crashing down on their heads.
Most commonly a Chapter becomes involved in a campaign when the local government sends out a distress call and the Chapter decides it's worth them getting themselves involved, whether due to some supply issue (the planet supplies them with arms, ammunition of food), because of some ancient pact, due to a sense of honour (we failed a mission, and that failure led to the planet being at risk) or because they had nothing better to do and the fighting would be good practice.
Sure the High Lords can give guidance, warnings and make requests of the Astartes, but at the end of the day if a Chapter decides not to heed an order from Terra there's nothing the High Lords can do about it. That's one of the reasons the Dark Angels have managed to get away with their activities for all these millennia.
The catechism you quoted is part of the Imperial Creed, used by the Imperial Guard, and not part of any Chapters' creed that I'm aware of. My information may be a little out of date though, so feel free to correct me on that point.
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