They shall be my finest warriors, these men who give themselves to me. Like clay I shall mould them and in the furnace of war forge them. They will be of iron will and steely muscle. In great armour shall I clad them and with the mightiest guns will they be armed. They will be untouched by plague or disease, no sickness will blight them. They will have tactics, strategies and machines such that no foe can best them in battle. They are my bulwark against the Terror. They are the Defenders of Humanity. They are my Space Marines and they shall know no fear.
-The Emperor of Mankind
The Adeptus Astartes (commonly known as Space Marines, and colloquially as Angels of Death) are one of the most elite and feared fighting forces in the Imperium of Man. The primary unit of organisation in the 41st millennium is the Chapter, a self-contained army fully equipped with its own transport, non-combatant support staff, etc. There are around a thousand Chapters, each comprising a thousand Space Marines. There are far too few Space Marines to form the Imperium's main military forces; instead they operate as highly mobile strike forces. They are entrusted with the most dangerous missions, such as lightning raids behind enemy lines, infiltration, and tunnel fighting. Through selection, training, and conditioning, Marines are physically, mentally, and spiritually superior to any other Imperial soldier.
Organisation
They shall be pure of both heart and strong of body, untainted by doubt and self-aggrandisement. They will be bright stars in the firmament of battle, angels of death whose shining wings bring swift annihilation to the enemies of man. So shall it be for a thousand times a thousand years, unto the very end of eternity and the extinction of mortal flesh.
-Roboute Guilliman
The Adeptus Astartes is comprised of autonomous Chapters, each of which is a complete army in itself, possessing its own transport, Warp-capable spacecraft, non-combatant personnel, and fortress-monastery based on a planet or fleet. There are roughly a thousand Chapters, each led by a Space Marine with the rank of Chapter Master. A Chapter's fighting force numbers a nominal 1000 Marines, divided into ten companies, each of which is commanded by a Captain. Companies are further divided into squads of ten battle-brothers, each commanded by a Sergeant.
The Chapter Masters owe their allegiance to the Emperor. Ultimately, the Chapter is subject to the orders of the highest-ranking among the Adeptus Terra, although only in a general sense.
Chapters are monastic communities in nature; for most of the Marines, the Chapter is their world, and the only contact they have with outsiders is in battle.
In Society
The average Imperial citizen often views the Astartes as gods-among-men, being subjects of myth and legend. Alongside this, the Ministorum venerates them as the God-Emperor's descendants and greatest heroes who birthed the Imperium. It is a grave religious taboo to even touch a weapon meant for the God-Emperor's Angels of Death. The common person might be taught the names of the greatest Space Marine heroes and their greatest deeds, as they are retold from millions of pulpits across the entire Imperium.
Very few mortals will ever see a Space Marine, as their numbers are trivial compared to the seething mass of humanity. The best chance an average human will have to see one will be if cataclysmic forces threatening the greater Imperium come to their world, or if they are part of similar Imperial forces sent to fight in such catastrophic wars. The subsequent chances of then conversing with an Astartes during such engagements are equally minute. A common human who does interact with even the lowliest of battle-brothers are likely to be awestruck in the extreme, unable to speak and forgetting themselves. They will typically be held in high regard and shown great deference, even the highest-ranking of officials bowing their heads in respect.
However, the reaction of extreme abasement and divine awe isn't universal to every human. Those who engage with Astartes with some degree of regularity, whether it be a Chapter Serf, military commander or Inquisitor, will grow accustomed to the presence and mannerisms of these transhuman brotherhoods. Demographics above or on the peripherals of Imperial society, such as nobles or hive scum, or others who largely disregard the frippery of Imperial propaganda might see the Astartes for what they are, tools of war. Some might not offer them any more fear or respect than that due to an expert soldier with a well-crafted weapon.
