This is made clear in Canto IV where Dante and Virgil meet the great Greek philosophers such as Socrates, Plato and Lucan.
These philosophers are in the first level of hell which is really more like a purgatory. Because they were born before the birth of Christianity, they did not have the opportunity to recognize Christ as their Lord and Savior so they live in a state of Limbo. When alive, they dedicated their lives to wisdom and thought rather than religion and did not have the foresight that the patriarchs of the Old Testament had in embracing faith that one day He would come. As a result, their afterlife is a world in which reason provides some illumination, but the atmosphere is mostly dark without God’s light. However, the souls are comfortable and content in being able to share human ideas and thoughts with other great thinkers.
This section of the Comedy firmly establishes Dante’s ideas that the only way to receive God’s blessing is through Christian faith. Even though the ancient philosophers were highly respected thinkers who led very virtuous, ethical and moral lives, they are still trapped in Limbo because they never even considered the possibility that Jesus would one day come to redeem them all. This is made clear when Virgil tells him that a person “in power crowned” came to Hell and took away all the Old Patriarchs who believed that the Messiah would come even though they died before he appeared. Thus it doesn’t even matter if you have lived the kind of life prescribed in the Bible, you still can’t get into heaven unless you were willing to have