What is the moral of A Tale of Two Cities, and how are the two cities significant?

The introduction to the moral of A Tale of Two Cities begins with the opening lines: 

"It was the best of times, it was the worst of times, it was the age of wisdom, it was the age of foolishness, it was the epoch of belief, it was the epoch of incredulity, it was the season of Light, it was the season of Darkness, it was the spring of hope, it was the winter of despair, we had everything before us, we had nothing before us, we were all going direct to Heaven, we were all going direct the other way."

The moral of the book is that there is duality in the world.  Where there is light, there is also darkness.  Right and wrong have very much to do with who writes the history and what the situation looks like when the blood hits the ground.  Because of this duality, it is important to be wary of whom you trust and causes for which you fight.  

The book deals with both London and Paris from 1775 forward.  For both England and France, this was a time of revolution.  England (King George III) was experiencing the beginnings of the American Revolutionary War and likewise Louis XVI is dealing with the social unrest of the people in France.  Both George and Louis are alike in their reactions to the situation (that they cannot understand why their sovereignty is being questioned); however, the results of the revolutions are polar opposites.  England emerges from the American Revolution with minimal damage, almost stronger than they were before, while France is fractured socially, morally, and the government is in shambles.    

Dickens’s points out the differences between London and Paris.  England is focused on ghosts and psychics while France is fixated on the religious leaders in order to evade torture and death. He also compares France and England, concerning their justice systems.  France's harsh justice system is mirrored with England's relatively relaxed system.  England finds itself overrun with bandits and rioting prisoners with very little consistency and order in the judicial system.  France on the other hand is strictly ruled through the new justice system the people demanded.  While they claimed that it would be impartial and better than the system before, instead it is the polar opposite.  Dickens does however, point out that even though England’s court system does have its flaws, it is far more stable than France’s justice system and does produce fair rulings.

In the end, the moral of the story is that a person must be responsible for their own choices, the people they trust, and the actions that they take.  Because of the duality that exists in the world, the only truth is in who you are and what you do, yourself.  Everything else is susceptible to corruption.  Because of the duality that exists even in yourself, it is important to weigh and be responsible for your actions.