At the end of Paradiso, when Dante looks upward to see the face of God, he sees three circles instead:
appeared to me; they had three different colors,
but all of them were of the same dimension;
one circle seemed reflected by the second,
as rainbow is by rainbow, and the third
seemed fire breathed equally by those two circles. (Par. XXXIII, 115-120)
Traditionally, these have been interpreted to be the Holy Trinity – the Father, the Son, and the Holy Ghost. Each of these makes up an equal and immutable part of God. This is represented by their "same dimension[s]." The Son (Jesus Christ) proceeds from the Father (God) as we see in "one circle seemed reflected by the second," and the Holy Ghost, according to scholar and translator Mandelbaum, "is the fire of Love breathed by both."
But there's something interesting about the second circle: "within itself and colored like itself, / to me seemed painted with our effigy." "Our effigy" is the image of a man. So the circle of the Son (Christ) has a picture of a man inscribed. This makes sense because Christ was both man and God at the same time. In fact, this is a point of great controversy: how can Christ be all man and all God simultaneously? Quick answer? We don't know. It's called the mystery of the Incarnation and it's this mystery that Dante contemplates at the end of the poem. How can one see the outline of a man in the circle if the man is the same color as the circle? Conventional logic would tell us the man is camouflaged into the circle, but Dante can see him here. In our world, that's simply not possible. But that's the whole point of the "mystery." As human beings, we cannot understand it, but must accept it on pure faith.
Dante, however, gets special treatment, because he's gone on this long hard journey, has purged himself of pride, understands Divine Justice (to the extent which man can understand it), has perfected his vision, and now gets to see God. Because he has begged God to let him remember what he has seen to help him on his poetic mission, God grants him understanding of this last mystery. It comes in a burst of light, but we don't get to see it. Such knowledge is not fit for mere mortals. We say too bad, but that's just the way it works.