Simon the Magician (“Simon Magus”) was the person in Acts who tried to purchase the gifting of the Holy Spirit for gain (Acts 8:9-24). Since then, persons who try to buy/sell ecclesiastical favors/positions are known as simoniacs.
In ditch three, the simoniacs are placed upside down in holes which Dante says are the size of “those in the font of my beautiful San Giovanni built to protect the priests who come to baptize.” Thus are those who mocked the priesthood themselves mocked. As they stuffed their pockets with ill-gotten gains, they are now stuffed into holes themselves. The reference seems to be to features in baptismal fonts that kept the priests from being mobbed during the annual baptism times. Apparently Dante (in real life) had rescued a drowning child who got caught in one of these fonts by breaking it. He was accused of sacrilege, and asks that this account suffice to exonerate him. Unfortunately this font style / feature has been difficult to prove (and some believe the few alleged remaining examples are not what Dante was talking about either).
As if being stuffed upsaide down into a hole in the black rock of the Inferno was not bad enough, the simoniacs feet are also burning with an “oily fire” (note the double reference to Holy Spirit symbolism – Dante, you are the man!). The flames are worse for the worse sinners (yet again showing the concept of corresponding punishment). Eventually other simoniacs will come along and stuffed into these holes, at which time the sinner is pushed through the hole into crevices in the rock. This, in fact, is what the worst of the simoniacs, Nicholas III (a Pope!) is waiting for Boniface VIII (another Pope) and one after him, Clement V (the current Pope of Dante’s day who was involved in moving the Holy See to Avignon).
Virgil carries Dante down a path that would have been difficult for a goat to tread so that he may converse with this simoniac Pope (in the manner of a priest giving last rites to assassins who were buried upside down). For the second time in the Inferno, Dante himself delivers stinging rebuke. His litany against this Pope is startling given his previous timidity, and Virgil loves it! As Dante winds down, he makes an interesting distinction between Popes and the Papacy when he says he would have used even harsher words but for his “reverence for the Great Keys you held in life.” Following Roman Catholic thinking, for Dante the holy office is not itself threatened by the evil men who sometimes hold it.
During his diatribe, Dante mentions holy men of old who did not sell out for the ministry given them, such as Peter and Matthias (“Matthew”). Matthias was the disciple who was chosen to replace Judas Iscariot as the twelfth apostle (Acts 1:15-26). No more is heard of him in the New Testament. Tradition does not help much either. Clement of Alexandria says some identified him with Zacchaeus; he is sometimes identified as Barnabas or Nathanael (cf. Gospel of John) as well. According to various traditions he may have preached in Ethiopia, Judea, Cappadocia, or the Caspian Sea area.
Dante continues by identifying this corrupt Pope as the Woman of Revelation 17 (who seems to stand for Pagan Rome according to John) and reviling the greedy Pope himself as an idolater:
“Gold and silver are the gods you adore – In what are you different than the idolater, save that he worships one, and you a score?”
Dante finishes by decrying the wealth associated with Church offices since Constantine’s time, and is then carried by Virgil back up to the bridge over the next ditch.