Justinian and the other souls depart, singing — and Dante is left with a doubt he is almost embarrassed to voice, since Beatrice, reading his mind, voices it for him. The question Justinian raised was this: if Rome's punishment of Christ was just (the punishment for the sin of Adam), then how could the destruction of Jerusalem by Titus — the punishment of those who carried out the Crucifixion — also be just? Justice avenging justice seems paradoxical. How can the instrument of legitimate punishment itself be punished?
Beatrice's answer is a compressed treatise on Atonement theology. The Incarnation was necessary not because God could not forgive by other means — God's mercy is unlimited — but because no other means would fully restore human dignity. Adam's sin was not merely a transgression; it was a disordering of human nature, a rebellion of the will against the good. Restoration required that human nature itself, from within, make an act of perfect conformity to God's will — which no fallen human could manage. Only if God became human could a human act be infinite in its value. So the Incarnation was the only way to satisfy both justice (the debt must be paid) and mercy (humanity must be restored). On the Cross, the human nature of Christ suffered a just penalty for human sin; the divine Person whose human nature it was made that suffering infinitely valuable. Both aspects are true simultaneously, without contradiction.