Rose as symbol of innocence
In an early letter, the amoral Marquise de Mertreuil describes the fifteen-year-old Cécile as "a perfect rosebud". Cécile, who was raised in a convent as was often the case for young women of her social class, begins the novel as a very innocent young woman. She is aware that she is engaged to be married, but is so clueless about the world that she mistakes her shoemaker for her fiancé. The rose also appears in the name of the elderly Mme. de Rosemonde, who is aware of her nephew Valmont's depravity but who naïvely allows him access to her home while two vulnerable young women (Cécile and the twenty-two-year-old Mme. de Tourvel) are also guests in her home. Mme. de Rosemonde has an optimistic view of the world and the people in it, and gives everybody the benefit of the doubt. In so doing, she enables bad people to do bad things.
Key as symbol of trust
In the very first letter, Cécile describes the freedoms and trust that she has been given by her mother. Cécile believes that she is being treated almost like an adult woman, and that she has her mother's confidence and trust in all matters. (This is not the case.) Among the freedoms and luxuries permitted to her, which did not exist in the convent, is a lovely little writing-desk to which she keeps the key. The fact she is trusted to have private space and correspondence not open to adult inspection thrills her. However, the door to her room in the Rosemonde household is locked every night to prevent any possible threat to her safety, possibly because the predatory Valmont also has access to the home. When Valmont indicates that he needs the key to Cécile's room to deliver letters from her crush Danceny, Cécile initially refuses to steal the key because she believes that it is wrong to violate the trust of her mother and her hostess by stealing a key and giving it to someone who is not entitled to it. Valmont pressures her to steal the key and give it to him, and Danceny (who does not know the details of Valmont's plan) pressures Cécile to do as Valmont says. Valmont later uses the fact she stole the key and gave it to him as a way to keep her from screaming for help as he lets himself into her room at night and "seduces" her (although his actions are considered rape by modern standards).
Money as symbol of approval
Valmont is the designated heir of his wealthy aunt, Mme. de Rosemonde. Although she knows about his libertine ways and is fully aware of the way he seduces and abandons women, she believes he is entitled to inherit her entire fortune after she dies. Secure in his access to massive wealth, Valmont has no reason to curb his behavior. Indeed, he abuses his access to Mme. de Rosemonde's house to advance his own predatory interests involving two of her guests.
One of the reasons Danceny is not as desirable of a husband for Cécile as the older, less attractive Comte (Count) de Gercourt is because Danceny is not wealthy. He is of the aristocratic class, and is a knight of Malta, but he does not personally possess an aristocratic title, hereditary lands, or significant amounts of money. Indeed, he is making his living by teaching music to Cécile while she lives at the de Rosemonde house. Cécile's mother, Mme. de Volanges, eventually withdraws her objection to Danceny because she realizes Cécile's inheritance from herself and her father will be more than sufficient to support their household.
One of the ways in which Valmont wins over Mme. de Tourvel is by giving money to the poor and to charitable causes. She interprets this act as tangible evidence that the noted libertine has changed his ways.
Cross as symbol of struggle
Although some of the characters in the novel are purportedly quite religious, the Christian cross is not used as a religious symbol of sacrifice or divinity. Instead it symbolizes a struggle. Danceny writes to Cécile about the "crosses (he) shall meet" and the difficulties he anticipates in becoming her husband. Likewise, a lawyer by the name of M. Bertrand who is representing her against Danceny after Valmont's death describes the "crosses" borne by his employer Mme. de Rosemonde as having had an ill effect on her health.
Letter as symbol of commitment
To put something in writing is to create permanent physical evidence of it that can survive apart from the person who wrote it. Therefore, to put expressions of love, especially forbidden love, into a letter is to take a significant risk. In setting her challenge to Valmont, the Marquise de Merteuil requires him to obtain letters from Mme. de Tourvel confirming the two of them are in fact having sexual relations.
There's plenty of rumor floating around the novel. The phrase "on dit que..." or "people say that..." does not have the same level of certainty as the solid evidence provided by a letter. Even the tense of the sentences containing "que" are typically written in the subjunctive or conditional, which in French indicates less than certain content.
