Sin and its consequences
Every soul in Hell is there because of choices made in life. The poem shows sin as self-defining: what you chose becomes what you are for eternity. Dante doesn't treat sin as abstract; he names names and assigns specific places.
The possibility of redemption
Purgatorio exists to show that sin doesn't have to be permanent. Souls who repented before death can still reach God. The theme pushes back against fatalism and insists that moral change is real and possible.
The limits of human reason
Virgil is the greatest poet and thinker of the ancient world, and he can only go so far. He can't enter Heaven. The poem argues that reason alone can't get you to God; you need faith and grace to complete the journey.
Political corruption and justice
Dante fills Hell with corrupt popes, greedy nobles, and treacherous politicians. He was exiled from Florence for political reasons, and the poem is partly his verdict on the world that wronged him. Justice in the poem is absolute where earthly justice failed.
Love as a moral force
Love drives the whole poem. Beatrice's love for Dante sends Virgil to rescue him. God's love is described as the force that moves the stars. But love also lands souls in Hell when it's misdirected or excessive. The poem treats love as the most powerful and most dangerous human force.