What is the overall theme of Dante's Inferno?

Learning how to attain salvation is the main theme of Dante's epic and subsumes all its other themes. The Divine Comedy is, therefore, a tale of the Pilgrim's education and, by association, the reader's. The reader follows Dante's Pilgrim through Hell in Inferno and learns with him about sin's pervasiveness. The torments of the sinners, who exist forever without hope of redemption or of an end to their suffering, graphically illustrate sin's consequences. As the reader and Pilgrim move through the underworld, the shades they see and speak with provide physical examples of and exemplary lessons on the seven deadly sins. At the end of Inferno, the Pilgrim and reader are better able to recognize sin in its various forms and to avoid committing it. Salvation and further spiritual education are impossible without such knowledge.

In the second section, Purgatory, the Pilgrim and the pilgrim reader move up the Mountain of Purgatory to the Garden of Eden at its peak. Along the way they learn the value of contrition and repentance, of having to suffer for causing suffering and for disobeying God. They learn this again by seeing and interacting with shades who represent the Seven Deadly Sins but who here exemplify the desire for contrition and repentance.

The learning process concludes in the third section, Paradise, where a plethora of saved souls appear to the Pilgrim and explain the workings of grace and God's love to him. In this celestial region, the Pilgrim takes a series of what we might call oral exams which test his growing knowledge. Schooled by his experiences in the three regions, having gained a firm understanding of sin and grace, the Pilgrim passes his exams and graduates to the vision of God. He, then, becomes a teacher, because he returns to earth with instructions to write about his experiences for the benefit of others.