Quote 16
As I inclined my head still more, I saw
that each, amazingly, appeared contorted
between the chin and where the chest begins;
they had their faces twisted towards their haunches
and found it necessary to walk backward,
because they could not see ahead of them. (Inf. XX, 10-15)
For claiming the superhuman (and thus unnatural) power of seeing the future, the magicians, diviners, and astrologers are subjected to an inversion of their natural form. Their faces, instead of gazing forward, are reversed on their shoulders so that they must face and walk backwards. Their sight has literally been reversed so that their sense of direction (and, possibly, time) is backwards.
Quote 17
When I had journeyed half of our life’s way,
I found myself within a shadowed forest,
for I had lost the path that does not stray.
Ah, it is hard to speak of what it was,
that savage forest, dense and difficult,
which even in recall renews my fear:
so bitter – death is hardly more severe!
But to retell the good discovered there,
I’ll also tell the other things I saw.
I cannot clearly say how I had entered
the wood; I was so full of sleep just at
the point where I abandoned the true path. (Inf. I. 1-12)
From this opening passage, one can see that the nature of sin (or "abandon[ing God’s] true path") is inherently treacherous because its path is "shadowed," "savage," "dense and difficult." As a road overcast with darkness, it limits Dante’s sight, both literally and metaphorically, making it difficult for him to ‘see’ the boundary between good and evil. Dante has already been tricked into his present predicament because he "cannot clearly say how [he] entered / the wood." Sin – deceptively innocuous at this point – has only made Dante "full of sleep," so that he cannot remember when he strayed off the straight road to God.
Quote 18
Then, as if penitent for my omission,
I said, "Will you now tell that fallen man
his son is still among the living ones;
and if, a while ago, I held my tongue
before his question, let him know it was
because I had in mind the doubt you’ve answered." (Inf. X, 109-114)
Dante, unlike the sinners, repents of his lie and contritely reveals the truth. His sense of shame, readers feel, is well-deserved because he has fallen to the spiteful level of the sinners. And it takes a conversation with a so-called "noble sinner, Farinata, to bring Dante to his senses.