When a group can write its own history, and when there is little competing data, it can twist the way in which later generations will view it.
The Classical Greeks of the “Golden Age” are often seen as an ideal, as a virtuous and noble group of people. Yet this is not true: leaders like Themistocles were comfortable with bribery, extortion, and human sacrifice; Thucydides tells us how Pericles gives a speech praising Athens for its morality and then tells us how the Athenians relied primarily upon dishonesty, intimidation, betrayal, murder, and cruelty for political power. Why do they have such a good image in history books, if they were so ruthless and corrupt? Some of the Greeks had a chance to write their own histories, and make themselves look good in the process; other Greeks wrote about how people should act, not about how they actually do act. The Greeks living during the Classical age would laugh themselves sick if they saw modern essays about the “noble Greeks”.
Remember, this is a society which embraced slavery, and notions of human inequality, to an extent which would shock an twenty-first century American; exclusion was one of the foundational concepts on which they based their society. Few societies have been more corrupt, sick, or depraved than Greece during the Classical age, and Athens in particular. Yet we remember then as the noble, democratic, virtuous Greeks!