changes as we transition from hell to purgatory. Although hell and purgatory are both earthly realms, hell was created by Lucifer's fall and is underground and for the damned. Now we are on the surface of the earth, the realm God created for the saved, and it is beautiful:

The lovely planet that is patroness
of love made all the eastern heavens laugh

All of the people on Mount Purgatory are on the path to full salvation. In washing Dante's face, Virgil is symbolically suggesting that purgatory is a place where souls are cleansed. It is the place where people do the hard work necessary to attain the highest realm: heaven.

The opening of Dante's work is reflective of the pilgrim who is searching for meaning in life.  The search for meaning revolves around the central questions of what defines individual identity and what choices individuals should make:  “Midway upon the journey of our life, I found myself within a forest dark, for the straightforward pathway had been lost.”  The "forest dark" is reflective of a condition in which individuals struggle to make decisions about the paths between good and evil.  The theme of good and evil is critical to Dante's work.

The layout of the inferno is predicated upon the choices made between good and evil.  Individuals who choose the latter are condemned to different layers of hell.  The pervasiveness of sin is a critical element within this.  Good and evil are essential themes in Dante's work because they provide a very basis for how individuals should live their lives.  Sin and the nature of human transgression is not embedded within the human psyche. Dante offers a view of hell whereby individuals are responsible for the choices they make.  There is good and there is evil and Dante sees the sum total of human actions are reflective of the choices that are accordingly made.  The failure to understand this condition is where sinful actions, choosing bad over good, condemns those who Dante and Virgil meet on their guide through the underworld.    It is through good and evil that humans can make active choices about the lives they live.  Emergence into the realm of Paradiso, to the domain where Beatrice fuses all opposites, can only be understood through the recognition and acknowledgement of good and evil.  The emergence from a "forest dark" is only possible through the recognition of good and evil.  In Canto III, Dante writes about the experience though which humans recognize the role good and evil play in their lives:  "And when his hand he had strech'd forth/ To mine, with pleasant looks, whence I was cheer'd/ Into that secret place he led me on."  Being led on, Dante, the pilgrim, understands the role that good and evil play in the formation of the human soul.  In this way, good and evil are relevant to Dante's The Divine Comedy.

As other answers here have noted, Dante's Divine Comedy seems to contain a catalog of themes, but as a quest story, its overarching one is the individual's journey to salvation. Salvation, in the medieval Christian sense, means union with God in Paradise, but along that journey are several moments that affirm what salvation requires, and this systematic poem is constructed on these moments of supporting themes.

At the end of each of the three epic books, Dante sees light, so we can understand the theme involving enlightenment. In Inferno, this light seems to be the stars, but in Paradisio, he recognizes that what he had glimpsed are actually the souls in Heaven reflecting God's light. Enlightenment comes, in a somewhat Platonic (e.g. "Allegory of the Cave") sense, from turning away from shadows or false reality to see the nature of human sin and virtue as they seem from a Divine perspective.

In Inferno, for instance, Dante the Pilgrim must confront the contrapasso of multiple sins in order to see what they really are. Each sinner receives a punishment that metaphorically represents the precise error within the sin. The sinners get what they really wanted all along, and they are punished by the knowledge that they chose the wrong thing. The knowledge as well as the contrapasso create the punishment, for in the Inferno, they cannot make another choice and are trapped by their own malevolent desires. The sin of lust, for instance, leads to being swept eternally in a hurricane. This perversion of real love, which uses others's bodies for one's own pleasure, involves a desire for the false feeling of being swept away by passion, as Franscesca describes. Canto by canto, Dante offers images of cosmic irony showing that sinners have loved the wrong things or loved incorrectly. In doing so, they distort their appetites, their passions, and their intellect, committing sins of excess, malice, or fraud. Dante the Pilgrim must come to an understanding of the nature of each of the sins he encounters in order to see the ugliness of each one, knowingly reject it, and move on. In this way, one could say that the two important themes within the Comedy are right knowledge (wisdom) and love—attributes Dante gives to God early in Inferno. As Dante journeys through Purgatorio, he sees souls cleansing themselves of small distortions in their knowledge and love, and in Paradisio, souls are fully aligned with the virtue they most love, thus placed in a corresponding circle.

A secondary theme, but one that Dante seems to have been highly invested in, involves the role of poetry or art to bring the soul to salvation. The poem repeatedly meditates on its own practice. In Inferno, for instance, Dante the Pilgrim seems mightily troubled when he encounters sinners damned through art—Franscesca was a bad reader, Brunetto Latini sought to "live" through his creative work the Tresor, but he ends up among sodomites whose fertility is squandered, and Ugolino uses narrative as a form of revenge. Each of these are dangers that Dante the Poet faces as he embarks on his own poetic journey through the Comedy and a recurrent temptation he must overcome to produce an epic that gives both fame and spiritual immortality to the poet.