Symbolism, Imagery, Allegory
While the favored insult of the Marquise is childishness, the Vicomte de Valmont prefers the sting of ironic terms of godliness. While writing to his former lover the Marquise, he regularly refers to the religiously devout Madame de Tourvel with exaggerated terms like "Heavenly Devotee" (1.44.2) and "Adorable Devotee" (1.44.15).
When writing to Tourvel, however, he lays on the religious imagery. He describes his desk as an "altar consecrated to love" (1.48.1). He asks her to hear his prayers and witness his tears (1.58.5).
Madame de Tourvel is true believer. For her, to be godliness is everything, which is why her lust for Valmont is unbearable for her. For a time, she keeps her faith. She attends Mass, prays fervently, especially when most tempted, and tries to bring Valmont back to the faith. As a sign of her fall from grace, she adopts the idolatrous language of her devilish seducer. Valmont replaces God as her one "judge" (4.128.4).
Her death, while tragic, isn't cynical. She returns to her faith, asking God to pardon Valmont and receiving the last rites from Father Anselme. Her return to the convent symbolizes her wish to return to godliness as the only thing that can save her now.