Dr. Manette's heroic return to France in order to intercede for Charles Darnay/Evremonde is in harmony with the theme of redemption in "A Tale of Two Cities." For, it is because of his having suffered in the Bastille, a political prison, that Manette is respected by the French Revolutionaries. His suffering--like Christ--is what earns him the respect of the masses; it is this suffering that lends him the opportunity and credibility to redeem both himself and his son-in-law. And, having redeemed the sins of his imprisonment, Manette can now bury his resentment for Darnay and not have to repress the old memories and resentment.
Since Dr. Manette's intercession for Darnay is a supreme act of charity towards the son of the brothers responsible for his being put into the prison of the Bastille, Manette's action contributes greatly to another theme, the brotherhood of man. Dickens suggests here that every evil will be brought to an earthly justice. In fact, it is no coincidence that Darnay's family name is Evremonde, with the first part suggesting the English word every and the last part, monde, suggesting the French word monde which means world, or all persons. (tout le monde=everyone). So, Darnay becomes a type of Everyman for Dickens. In his world, Dickens shows the reader that there is room for grace and redemption as the later act of Carton,an act anticipated by Manette, will show.