Everybody in A Tale of Two Cities seems to have secrets: Dr. Manette's forgotten history detailed in his secret letter; Charles's secret past as an Evrémonde; Mr. Lorry's tight-lipped attitude about the "business" of Tellson's Bank; Jerry Cruncher's secret profession; and Monsieur and Madame Defarge's underground activities in organizing the Revolution. In part, all this secrecy results from political instability. In the clash between the French aristocracy and revolutionaries, both sides employ spies to find out their enemies' secrets and deal out harsh punishments to anyone suspected of being an enemy. In such an atmosphere, everyone suspects everyone else, and everyone feels that they must keep secrets in order to survive.
Through the secrets kept by different characters, A Tale of Two Cities also explores a more general question about the human condition: what can we really know about other people, including those we're closest to? Even Lucie cannot fathom the depths of Dr. Manette's tortured mind, while Sydney Carton remains a mystery to everybody. Ultimately, through Lucie's example, the novel shows that, in fact, you can't ever know everything about other people. Instead, it suggests that love and faith are the only things that can bridge the gap between two individuals.