The Dream in Chapter 9.
In the ninth canto of the Purgatory there is a prophetic passage where Dante is having his first dream during the first night, of being hunted down from above by an Eagle and lifted up into The Fire. There they both burn and the intensity makes the Pilgrim wake up, partly in fear.
The literal explanation that is then given by Virgil is that the Pilgrim has been lifted up from the Valley of Princes and up to the Gate of Purgatory by St. Lucia while he was sleeping, but there is also a much deeper symbolism in this passage, one which has been interpreted in several different ways in the various commentaries of the Divine Comedy.
After a correspondence with Professor Raffaele Giglio at the Department of Italian Literature at the University of Napoli, Federico II – the interpretation seems more clear: that the fire is a symbol of the Divine itself, with references to how this is portrayed in the Biblical Stories of Moses, which also connects the Purgatory once again to the Book of Exodus. This theme is also explicitly introduced in Chapter 2 when the souls are arriving at the beach singing “In Exitu Israel de Aegypto” in one single harmonious voice.
The theme of fire and unification is thus prophesizing the very ending and aim for the whole Comedy, to reach up to the Divine in the Highest Sphere of the Heavens and experience a mystical union with the help of the Grace of the Virgin Mary, after the prayers of the great Mystic St. Bernard.
From the correspondence with Prof. Raffaele Giglio, translated from Italian:
The presence of the “Fire” and the motif of the kidnapping, even though in the dream, makes us able to anticipate that which was the aspiration of Dante; to reach the Divine, which in the Biblical Stories was represented in general as the burning bush, as vivid and living Fire.
Dante in his proceedings has had to confront and surpass many barriers of fire to continue his journey. The fire, in my interpretation, symbolizes the burning desire of the pilgrim towards that biblical image of the Divine. The fire is an expression of strong desire; and in the dream Dante has brought to completion that which burns inside of him: unifying himself with the Divine Fire. The image is symbolic, and could signify the aspiration towards this Fire-Divinity, which now in a dream seems to him to have happened.
Through this dream at the transition point between Ante-Purgatory and the real Purgatory, Dante as a writer is stretching out the full canvas for the story, and in many ways his whole purpose for writing the Comedy – to show a Journey towards the Divine through a personal example, as inspiration and for imitation for us as Readers. The first idea and experience of this is felt through the burning image – which the Pilgrim is not ready for yet as his spiritual growth is in the beginner stages, and thus it snaps him out of his dream. But the feeling lingers after he wakes up, and a little seed is planted in him for the further climb, and for the ascent into the Heavens.