"Prisons we choose to live inside", by Doris Lessing. Try to check out the Chapter "You are damned, we are saved".
"I was brought up in a country where a small white minority dominated the black majority. In old Southern Rhodesia the white attitudes towards the blacks were extreme: prejudiced, ugly, ignorant. More to the point, these attitudes were assumed to be unchangeable and unalterable, though the merest glance at history would have them (and many of them were educated people) that it was inevitable their rule would pass, that their certitudes were temporary. But it was not permissible for any member of this white minority to disagree with them. Anybody who did faced immediate ostracism: they had to change their minds, shut up, or get out. While the white régime lasted - ninety years, which is nothing in historical terms - a dissident was a heretic and a traitor. Also, the rules of this particular game demanded that it was not enough to say "So and So disagrees with us, who are the possesors of the evident truth". It had to also be said "So and so is evil, corrupt, sexually depraved", and so on".