by Richard Eldred, Daily Sceptic:

In the old Soviet Union, citizens were not required to be a member of the Communist Party. But if you were not, you could never expect to rise far professionally or socially. You would never be the head of a department in university, a factory manager, much less the General Secretary. They were always recruited out of the party.

Party membership was proof of loyalty. It was a demonstration that you were willing to put loyalty over morality. Rising high in the party also meant that others in the ruling class likely had something on you. No one gained power without other powerful people knowing of your grim deeds. That way there was mutual trust, or, to put it another way, mutual blackmail.