Quote from : Horus Rising Book

The Emperor, you see, protects... He protects mankind, through the Legions, through the Martial corps, through the war machines of the Mechanicum. He understands the dangers. The inconsistencies. He uses you, and all the instruments like you, to protect us from harm. To protect our physical bodies from murder and damage, to protect our minds from madness, to protect our souls... There are insane dangers in the cosmos, dangers that mankind is fundamentally unable to comprehend, let alone survive. So he protects us. There are truths out there that would drive us mad by one fleeting glimpse of them. So he chooses not to share them with us. That's why he made you... Remember, Garviel. The Emperor is our truth and out light. If we trust in him, he will protect.