The population of the Imperium is unimaginably vast. The Imperium's roughly one million worlds are inhabited by humans or human-descended abhumans. Although they are all citizens of the Imperium, citizenship confers no rights, only responsibilities.
Adepts are servants of the Emperor and are legally above simple citizens, possessing at the very least, the legal benefits of housing and employment within the Imperium. The difference between adept and common citizen is much more pronounced on Terra.