The religious belief in the God-Emperor of Manknd and the expansion of the Imperium of Mankind. To follow this religion is to obey the will of the Emperor and live under his righteousness. Furthermore the followers must seek out the heretic, the mutant, and the xenos and destroy them. Today there are not many followers of this belief but The Emperor has forseen the future of Mankind and Man's destiny to conquer the stars and The Emperor is the one to lead Man.
As said in the Imperial Creed 'The Emperor Protects'