As Dante and Virgil enter the cornice of the Wrathful, they are enveloped in a dark cloud of smoke, a "foul and acrid air" which chokes the senses. It might be worth comparing the images used in Purgatory with those used in the Inferno for similar sins. Dante had gone into more detail and description of the nature of sin in Hell; here in Purgatory he will give us simpler images. These are, after all, the souls of the Penitent, and so the effects of sin have not fully consumed them. Instead, what we have seen so far is a pattern of contrast: examples of vice and virtue placed before the readers' eyes. We see the contrast not only in the historical examples which are given on each cornice, but also in the particular prayers which are chanted or sung by the Penitents. On this cornice Dante hears the souls singing the Agnus Dei (Lamb of God). Our attention therefore is not to be fixed on the negative (the vice or sin being purged) but on the opposite virtue which is being held forth as an example to imitate. The Agnus Dei is an apt choice for this part of Purgatory, since it brings forth the image of Jesus who returns the wrath of his accusers and executioners with a plea for forgiveness and the meekness of the "lamb who is still before his shearers". And in comparison to the discord he encountered among the inhabitants of the Inferno, in this place Dante hears the souls singing in perfect harmony (XVI.21).
In the midst of one of Dante's conversations we have what might be a beautiful summary image of the work of purgation. Dante is beginning to answer Marco the Lombard's question, and he addresses Marco:
O you, creature, who purge your soul
in order to give it back to God...
This is the goal of the whole process: being cleansed in order to give one's soul back to God, from Whom the soul had its origin in the first place. This idea then allows Dante to introduce the topic of free will and destiny. If indeed "grace does not destroy nature, but perfects it", then part of that perfection involves the human faculty of choice. The relationship between grace and free will is either more simple (not more simplistic) or more complex than what Dante presents in this isolated canto. Yet it is a question which is with us perpetually. First, his new conversation partner is quick to dismiss some popular (then and now) "causes" of human action. Dante asks Marco about this, commenting that some people see cause, destiny, "in the stars" and others see it "on the earth" (63). Marco tells Dante that the "spheres" do not control destiny, for if this were so, then free will would be destroyed (68-71). No, he says, if the world has gone downhill, then the "cause is in yourself and only there". This, too, is then tied to the notion of justice: unless we are free agents in doing good or evil, then there is no justice in reward or restraint.
Dante gives us a brief overview of the development of human free will agency in the individual and in society. Since we are born with the capacity for free will, but not with the wisdom to make the right choices, we need to be guided by the restraint of law. And if the world has gone to hell in a handbasket, (as every generation believes it has) then the fault is in the evil will of the leaders who are organizing either the religious or secular society, it is "not Nature that has grown corrupt in you" (Musa's trans, XVI.105).
This then leads to Dante's last point in the canto. The confused and inappropriate blending of secular and sacred powers by the church of Rome has put Pope Boniface VIII into the Inferno.