Dante has been losing his way. He needs to tour Hell so he can get back on the righteous path. (Inferno in its entirety.)
Dante needs help in a bad way because he is lost in a dark wood, symbolizing his corrupt moral state. As we learn later in the Comedy, Beatrice—the love of Dante's life—has died and this is part of the reason he is plunged into despair. He has sunk so deep into sin that he has attracted the attention of the Virgin Mary herself, whose compassion leads her to try to save Dante. We know this is the initial situation because Dante is in the darkest part of his life In Hell, Dante learns to harden his heart to the suffering souls and learns to condemn them for the sinners they are.
Could all of Dante's meanness to the sinners be a result of his sin of pride? Proceed with purgation. (Purgatorio Cantos I-XXX)
Having survived Hell, Dante comes face to face with his first real conflict: he has committed the sin of pride. Remember all his holier-than-thou rhetoric against Florence and her sinners in Inferno? That comes partly from his pride. Thus, Dante suffers with the Prideful on the First Terrace, pulling his own symbolic share of weight. Though he doesn't purge his soul of pride, he recognizes the sin in himself and the need to address it. For the rest of his purgatorial journey, Dante remembers his sin and constantly makes tortured references to it.
Virgil disappears. Beatrice scolds Dante. Dante hangs his head. (Purgatorio Canto XXXI)
Dante has learned to trust Virgil, so when he disappears Dante feels as if he has lost a father. As readers, we know that pagan Virgil cannot possibly set foot in the holy Earthly Paradise, the former Garden of Eden.
To further complicate things, Beatrice has little mercy for Dante, quickly putting him through an emotionally harrowing confession in preparation for his dunking in the Lethe. Her accusations are all the more painful because they prove that Dante swerved from the true course even after witnessing the goodness of Beatrice. Dante is properly ashamed. As readers, we fear for Dante here, unsure whether he is worthy enough to continue on his journey.
Beatrice deems Dante worthy of proceeding into Heaven. She gives him his poetic mission. (Purgatorio Cantos XXXII-XXXIII)
After undergoing Beatrice's terrifying inquisition, Dante is deemed worthy to continue with his journey. Before heading to the river Lethe, Beatrice conveys God's message to Dante that his mission will be to observe all the happenings from this point forward, record them as accurately as he can, and bring this confessional back to Earth in the form of a poem.
We recognize this as the climax because all of Dante's suffering and learning is given a direction and reason. That Beatrice, his love, bestows it means even more to him because it brings his personal and spiritual life into harmony.
Dante ascends through Paradise to the Eighth Heaven of the Fixed Stars. He witnesses the re-ascent of Christ and Mary. There, he's grilled on his theological knowledge by St. Peter, St. James, and St. John. (Paradiso Cantos I-XXIII)
Having passed into the heavens, Dante goes along happily, learning theology until he confronts another test—much like the confession Beatrice put him through. Here, though, the stakes are much higher. These questions on Biblical theory test whether or not Dante is worthy of entering the Empyrean, where all the blessed souls reside. The three saints question Dante on his knowledge of the Three Theological Virtues: Faith, Hope, and Charity.
Dante answers all three inquisitors to their satisfaction and is allowed into the Primum Mobile. He learns angelology and ascends into the Empyrean to see the Celestial Rose. Beatrice disappears. (Paradiso Cantos XXIV-XXXI)
Dante has proven himself worthy. Woo-hoo. He ascends into the Ninth Heaven of the Primum Mobile and eventually into the highest realm of Heaven, Empyrean itself. His observation of the two hosts— the angels and the blessed souls—is interrupted when he finds that Beatrice has disappeared.
As readers, we say "Whew! He's made it!" and it seems as though everything is happily winding down for Dante, but suddenly (when something as unexpected happens as Beatrice disappearance), we're left scratching our heads and wondering what will happen next.
St. Bernard replaces Beatrice and prays to the Virgin Mary to God on Dante's behalf. While he prays, so does Dante. He sees the Holy Trinity, then is granted the vision of God himself. Triumph! But we can't see what he sees. (Paradiso Cantos XXXII-XXXIII)
Beatrice's disappearance echoes Virgil's disappearance in Purgatorio XXX, and is the ultimate test of Dante's faith: he loses his love yet again. But not really; she's up with the blessed and smiles down on him. She even joins in the sung prayer to Mary on Dante's behalf.
St. Bernard's purpose quickly becomes clear. Nobody can see God without going through Mary first. As Mary's devotee, he prays to her on Dante's behalf. When Dante is granted the gift of seeing God, the implication is that he is blinded by the burst of light that ensues. We can't see what follows.