Dante & Virgil continue to weave their way through the souls of those whose sin was excessive love of worldly things - avarice. The poet summarizes the process of repentance on the mountain with a wonderfully simple image:
the mass of souls whose eyes were, drop by drop,
shedding the sin which occupies our world
How is sin overcome? By sheer willpower to do good? By an abrupt turning (conversion) to what is good and virtuous? Well, at least in this canto, Dante gives us the image of the relationship between the sorrow of repentance and the progress toward virtue. Perhaps we live in age when the idea of actually being sorry for one's faults is a bit gauche. Not-the-sort-of-thing-we-do, at least at our church.
Mount Purgatory reminds us that there is, for most of us, no shortcut to virtue. Mind you, those of us who are notoriously fault-ful will find that many of our closest companions wish that we would at least try to hurry along the path. Nevertheless, I suspect that the tears of repentance are a form of mourning or grieving, as one "dies to sin", to borrow New Testament language.
The souls who are on this level sing a two-fold song. During the day they recite examples of the humilty of Mary who gave birth in a lowly stable, and the generosity of Nicholas (yes, the original Santa Claus). But as night falls they cry out examples of avarice and greed. Thus Dante and Virgil see virtue and vice juxtaposed as day and night, and the reader can easily grasp what the poet is conveying.
Toward the end of the canto Dante feels the mountain shake and tremble, accompanied by a shout so loud that it startles him. This prompts Virgil to draw closer to him and reassure him with the words "You need not fear while I am still your guide". (XX.135) In the story of the poem, we learn that the exuberant shout of Gloria in excelsis comes as the mountain releases the soul of a penitent who is ready to "move up". While Dante does not immediately understand what is happening in the mystery of grace, the presence of Natural Reason (in the figure of Virgil) should be enough to keep him from fear.
"Grace does not destroy nature, but perfects it."