The Monk is set primarily in Catholic Spain. The Virgin Mary, or Madonna, is held in Roman Catholicism to have the power of intercession with her son, Jesus Christ, in order to give miraculous aid to those that petition her in distress. She has this 'magic' power given her proximity to Christ as his blessed mother and as the Queen of Heaven. She is a protector of good Christians.
The author, Lewis, has the beautiful Matilda provide Ambrosio with a picture of herself as the Madonna or Virgin Mary. The author here provides a mirror effect, where the natural order is reversed. What appears virginal and pure is secretly now carnal and corrupted by Matilda. Matilda has no intention of aiding Ambrosio: she has actually laid a trap for him. Another mirror image or inversion is that the seductress Matilda was initially introduced as a boy, Rosario.
Why is this inversion or mirror effect used? This inversion of the natural moral order creates a deep sense of shock and horror in the reader later on, when the devious nature of the whole plot is gradually revealed. The effect is similar to what a devout Catholic exposed to a Satanic parody of their faith in a black mass might experience (revulsion and horror). The sacred is thoroughly mocked by the wicked. The author uses these literary devices to create strong images and emotions.
A carnal seduction aided by a sacred icon of the Virgin is bad enough, but when we eventually find out Matilda is Antonio's sister and that she has intentionally added incest to the list of her brother's crimes so that Lucifer can damn his soul, the monstrous scope of Lucifer's plot to damn Antonio is realized, and the pathos of the story's gothic horror reaches a peak.