Good day, this is Zak Ludick here from Cape Town South Africa! I am bringing you more Warhammer 40,000 lore.
When it comes to the Imperium of Man, I have shown you Space Marines, Inquisition, Sisters of Battle and Imperial Guard... ahem Astra Militarum... and one thing that a person may discover a lot is a universal warcry of "For the Emperor!" as well as the phrase: The God-Emperor of Mankind.
Now... what is that all about?
Well, the whole truth is always shrouded in a bit of a mystery. The Emperor never called himself a God, but there was a church started by one of his sons (one that incidentally turned to Chaos) that began revering the Emperor as a holy entity and eventually a God. Given that the Emperor is Immortal and has been around for thousands of years.
It is said that the immensely psychic Emperor was created with the sacrifice of a large amount of psychic and wise people, amalgamating their powers to create the savior of mankind that will reunite mankind across the stars and fight off all the forces that threaten humanity with extinction.
Besides creating the Primarchs and the Space Marines, the unification of Terra and brining the Adeptus Mechanicus (remind me to to do a post about these guys) under his command as well... the Emperor went on a crusade o unify all the human worlds.
This was not even his greatest achievement that would ensure the survival of the Imperium even after him being mortally wounded and practically dead... and that is the function that he still serves in his undeath state on the Golden Throne.
The Astronomicon, the beacon in the Warp that is so strong that you can see the Emperor's soul shine inside the warp from across the galaxy. There is a special breed of sanctioned psyker/mutants called Navigators who have the ability to "see" inside the warp while they travel through it and navigate the galaxy with the help of the Emperor's shining beacon as a point of reference.
Without this beacon, the Navigators would not be able to tell where they were going and Imperial ships would not be capable of Warp travel. Thus the ships of Mankind would only be able to travel at regular impulse speed with no sort of FTL flight capability (Faster Than Light).
Without an FTL drive, the Imperium would be cut up into tiny bits, unable to respond to threats and planets and star systems would fall to the enemy without backup being available at any great speed.
Without the Emperor, the Imperium would surely fall... perhaps... there are SOME speculations but we shall cover this another time in another post!
Thank you for reading and have a wonderful day further!