Abandoning the Emperor is not the same as abandoning mankind...but as you were not there, I would not expect you to understand. Even those who stood against us would never have accused us of heresy. Your ancestors, your Primarchs...all would be ashamed of the dogmatic fools that you have become. Except perhaps Dorn, who knew nothing but dogma himself.

We are still weapons, brother. The difference is that we fight for ourselves, or for who we choose, and we do not deny ourselves access to tools that make us stronger. Bark whatever empty words you choose, my blinkered friend. Your death will come when we are returned.

PS. You can spot a legion warcry in his words, if you look carefully enough. Although when it comes to the whimsical vagaries of chaos, its hard to know what you can rely on.

Those of us Who are still loyal to the one true God shall stand as the bulwark of humanity! My faith shines the brightest When I crush every enemy of man! The Brothers I command Have sworn in the name of the Emperor that we shall never find ourselves wanting. Each and every one of us are honor bound to stop the spread of chaos in any way shape or form. I have served the Imperium for countless Millennium and in my life I have seen worlds put to the torch to stop the spread of the chaos gods. It may have costed millions of innocent lives but their deaths are forever entombed in the hulls of vengeance. With the Emperor as my witness i shall not yield or faultier before such heresy. CARRY THE EMPEROR'S WILL AS YOUR TORCH BROTHERS! BROTHERS LET US CLEANSE THIS WORLD OF CHAOS! FOR THE GLORY OF THE EMPEROR!!!

These heretics continue to spread their lies and temptations, they seek power for they feel insecure. They refuse to see that the emperor's blessing is the only power a warrior needs.

As always, you can justify yourself as many time as you wish but that wont change anything in my head because I have accepted my faith and my duty with utmost honor...and don't be so sure about my knowledge of Primarchs and their Legions, I can assure you that I remember time way before Warmaster's downfall to insanity...

Just going to slide into OOC for this post, as you've both raised an interesting point at the same time...

As I understand it, the only loyalist marines that are still alive from the days of the Heresy have;

a) Been interred in dreadnoughts (e.g. Bjorn the Fell-Handed)

b) Spent that time in the warp unintentionally (e.g. the loyalist remnants of the Space Wolves 13th Company)

c) Spent that time in the warp intentionally (e.g. Primarch Corax, if he's actually still alive)

The oldest loyalist Space Marine not falling into the above categories in the lore is Commander Dante of the Blood Angels, who has been kicking around for about 1,300 years. Given that the Horus Heresy was just shy of 11,000 years in the past, Dante doesn't even come close.

There's no example of a Space Marine dying of old age (although we know their bodies decay and become decrepit eventually), mostly since marines on active service don't tend to get as far as 'old age', but a combination of their hazardous existence getting them killed, seeking glorious last stands when their bodies become too worn down, or ending up being interred in dreadnoughts seem to be what occurs.

The traitor marines are somewhat different, because those alive in the Heresy days fall mostly into the b) and c) categories.

Because time passes differently in the warp, there are traitor marines for whom only 200 years of time has passed in their reckoning (e.g. Talos of the Night Lords), whereas 11,000 years of time has passed in realspace. And not all traitor marines have experienced the same length of time passing, since the warp isn't exactly predictable.

As a result, there are a reasonably large chunk of traitor marines still alive from the Heresy days, whereas there are only tiny handful of loyalist dreadnoughts and space marines lost to the warp in the same position.

TL;DR

If you're a loyalist who remembers the days of the Heresy, you're either a dreadnought or have been (or still are) stuck in the warp for most of the last 11,000 years.

If its the 'stuck in the warp' option, you might need a creative backstory about why your legion would accept someone who has been in the Eye of Terror (or other warp anomaly) for that long back into its ranks.

Alternatively, you need to give the existing lore the pre-Unity salute (but with middle finger extended) and come up with a reason why you're still alive and kicking that hasn't got you executed for heresy or confined to a metal box with barely any parts of your body still functioning.