This novel is significant because it is grounded in reality--not because it could happen but because it did happen. Of course, every character and action in the novel is not historically real; however, the motivations and consequences are based on the realities in both France and England.
As Dickens draws parallels between England and France in his opening chapter, having been greatly affected by his reading of Thomas Carlysle's The French Revolution: A History, he felt that England was equally as ripe for an upheaval as was France. For, with the Industrial Revolution, there was the creation of a threatening underclass. Much like the peasants of France, this underclass was starved, overworked, and overcrowded. And, as a social reformer, Dickens employed the truth--the "tale of two cities"--that is embedded in fiction as a warning to his society of what could happen.
It would be very difficult to disagree with this statement. Of course, we do need to remember that Dickens blurs the boundaries of truth and fiction by creating characters and placing them in a real-life historical context, but certainly the setting and historical events that are depicted in the novel, such as the storming of the Bastille, are historically accurate and true. In addition, it is important to remember that some of the characters, although they might not have actually existed as such historically, are representative types that sum up the best and the worst of the people they represent. So, for example, in Book II Chapter Seven, Monseigneur is made to represent the French aristocracy in all of their dazzling excesses:
Monseigneur could swallow a great many things with ease, and was by some few sullen minds supposed to be rather rapidly swallowing France; but, his morning's chocolate could not so much as get into the throat of Monseigneur, without the aid of four strong men besides the Cook.
This quote points towards the massive division of wealth experienced by France before the Revolution, and the way that the aristocracy were said to be "swallowing France" in their lavish excesses. Therefore this novel, although it is fiction, is set in a real historical context.