Dante first introduces the reader to the idea in the Vestibule, where the uncommitted are punished.Because they were uncommitted in life, they are forced to forever chase, but never catch, a pennant while being tormented by stinging insects.
According to the Christian Church of Dante’s time, such souls could not be saved (although Dante writes that several were saved by the personal intervention of Jesus, who led them out of Hell following his crucifixion), but Dante felt that such souls would be condemned least of all, suffering no physical tortures, only the mental anguish of knowing that they could not know God’s love.
Dante’s guide through Hell and later Purgatory is one of these virtuous pagans – the poet Virgil, who wrote the Aeneid.
Limbo is the only place within Hell that doesn’t apply.
After all, the only “sin” the virtuous pagan committed was something they ultimately had no control over – being born before Christ and/or dying without having been baptized.
At the base of the cliff is the Seventh Circle where the sins of violence are punished.