How the Reflection of Light Disposes our Darkness.
Canto XV steps forward through the final seven levels of Purgatorio as Dante proceeds through the seven capital sins. Dante and Virgil have just left the second Terrace of the Envious and have reached the third Terrace which represents the sins of Wrath. Throughout this Canto the theme of the importance, power, and necessity of God's Divine Love is illuminated through the splendor of light and its reflection. The emphasis on the reflecting power of this Divine Light is Dante-the-Poet's way of expressing the importance of spreading God's Love in order to overcome and purge ourselves of the darkness of our sins. Thus, the reflection of Divine Light exposes our darkness, and that same "intensity of light" will ultimately purify our souls, allowing us to enter into the splendor of God's eternal paradise. Virgil explains this to Dante in his second response to Dante's hunger for answers, saying, "the greater the proportion of our love, the more eternal goodness we receive" (Purg. XV, 71-72). However, this experience is clearly meant to be a process, not an instant gratification to appease one's own appetite for eternal bliss. The reader observes Dante's growing "hunger . . . for satisfaction now" (Purg. VX, 58), and in contrast there are several occurrences that symbolize a purging of Dante's sins and a foreboding of more to come. Canto XV visibly focuses on the process of becoming free from sin, underlining that, for Dante, there is definitely more to come. But the main point is that the process he must go through - a process for the reader as well - will involve an exposure of Dante's sins through the "intensity" of God's reflecting light.
Dante's process of purging commences at the very beginning of Canto XV with the significance of the location and movement of the "spera", the sun. The movement, or constant flow, of the sun across the sky is a reflection of Dante's own journey through purgatory.