every time he tries to say it.  He allows Charles to tell him on the morning of their marriage.

Dr. Manette understands how the news will affect him.  Anything having to do with his imprisonment sends him into shoe-making relapse.  He knows that this will happen if Darnay tells him anything about his past.  He asks Charles to wait, because then the young couple will go away on their honeymoon and not know what happens to Dr. Manette.  He wants Lucie to be in the dark about everything, so her wedding is not ruined and she does not worry about him.

The fact that Charles needed to tell Dr. Manette who he was so badly is a sign of his character.  He does not want to feel like he is marrying Lucie under false pretenses. He loves Lucie, but he does not want Dr. Manette to ever think he lied.  Marrying the daughter of one of his father’s victims is not an easy thing for him, but he is trying to escape his past.

Dr. Manette likewise tries to avoid hurting Lucie.  At first, he does not want to hear the news.  Yet he understands how important it is to Charles.  In one of the major themes of the book, he makes a sacrifice to hear the news, knowing he will relapse.  He recovers fairly quickly, showing his recovery is progressing.

Charles Darnay, on his return to Paris during the Revolution, is arrested for treason because he is an “emigrant.” This refers to those aristocrats who emigrated from France and went to England. By doing so, they forsook their wealth and estates that were left behind. Some had the foresight to place some funds with Tellson’s Bank in London, where many of them gather. When Charles returns to France, it is assumed that he has returned for his wealth, though in fact he has come back to help his servant Gabelle. He is held in prison until his trial, which portrays him as having betrayed France and her people (meaning the revolutionaries) by the Defarges and Doctor Manette. While imprisoned in the Bastille, Doctor Manette had written an account of the causes of his arrest and curses the name of Evremonde and all their descendants. Unfortunately, this includes Charles, so there are three witnesses against him. This is enough to sentence him to death.

Charles Darnay is a French tutor in England.

Charles Darnay's secret that he hides from all except his father-in-law before he marries Lucie, is that he was born into the French aristocracy, and in addition, is the nephew of the Marquis Evermonde, who we are shown is a particularly cruel and heartless example of the French aristocracy. We are shown Charles' great strength of mind in his rejection of the values of his uncle and in his decision to live a significantly poorer life working in London than he would be able to live in France. We are also shown his tremendous loyalty in his decision to return to France to try and rescue Gabelle at great personal risk.

Because Charles Darnay is a man of conscience, he is troubled by the abuses he sees aristocrats of his class, especially from his own family, perpetrating in France before the French Revolution. After he realizes that his evil uncle has plotted to have him tried and convicted of treason in England—a plan which was foiled by Sidney Carton—Darnay becomes distressed at being part of such a corrupt aristocratic family. As a man of integrity, he feels he can no longer be included in a clan that behaves so badly and uses its power to hurt and oppress others.

Therefore, although he is a wealthy aristocrat, Darnay gives up his titles and privileges to move back to London. He also gives up his family name, Evremonde, and takes on the humbler moniker of Darnay. In London, he uses his fluency in French to become a tutor in French. This is a far cry from his former life.

Dickens, who was a member of the middle class himself, makes several points through the exemplary Darnay. First, Darnay rejects the idea that being born an aristocrat makes him innately superior to others. Superiority, as Dickens shows, is earned, not bestowed at birth. Further, Darnay is portrayed as a good person for taking on an honest job and earning his own way rather than living on inherited wealth.

In an ironic twist of Fate, whereas Doctor Manette is first able to get his son-in-law released from his charge of treason against the state, Manette unwittingly figures into Darnay's recapture and imprisonment.  For, at first Dr. Manette is heralded as a heroic survival of political oppression in the Bastille, but upon his second efforts for Darnay, Manette is recognized as the physician who suffered by the hands of the Evremonde twins, one of whom is Darnay's father.  

In Chapter 4 of Book the Third, Manette appears before the court and Darnay seems at the point of being released, when

the tide in his favour met with some unexplained check, which led to a few words of secret conference

and Darnay, ne [born] Charles d'Evremonde, is returned to prison.  The secret conference that has been held involves the connection of Dr. Manette with the brother of Madame DeFarge, the young man mortally wounded after defending the honor of his sister, Therese DeFarge.  Unbeknownst to Manette and Charles d'Evremonde (Darnay), Madame Defarge has knit his name into her deadly cloth and has recognized Charles after his having come to speak on behalf of the tax collector for the Marquis d'Evremonde, Monsieur Gabelle, who wrote him a desperate letter.  She is in possession of a letter that Dr. Manette has written while he was in prison ( 1757-1775) that describes the atrocities committed by the Evremonds against her family.  And, as the son of one of the perpetrators, Charles d'Evremonde will be held accountable.

In addition to the vengence which Madame Defarge seeks against this aristocrat named Charles d'Evremonde, the bloodlust of the revolutionaries has also risen and the "new era has begun."  The Reign of Terror has begun: 

The deluge rising from below, not falling from above, and with the windows of Heaven shut, not opened!

Darnay did not want to reveal his family connections to anyone in England.  He had renounced his family because he disagreed with their methods, but that did not mean he would not still be considered guilty of being a member of the family, whether or not he personally wronged anyone.  Since there was obvious unrest in France, both countries were suspicious of anyone going back and forth between the countries without good reason.  Darnay, being a private person and trying to hide his ties to the St. Evremonde family, refused to give an explanation for these trips.  Since he refused to tell, it was assumed that he was up to no good.

There were many spies going back and forth from England to France.  The trip was an especially dangerous one if you were French and living as an expatriate in England.  Darnay also realized that revealing his family connections would only further condemn him.