(Canto 2, line 24)
"The greatest Peter" is Saint Peter, the first disciple of Jesus. He was appointed a successor or representative of Jesus, witnessed the transformation of Jesus and attended the Last Supper. He came to Rome via Syria and became its first bishop. Together with Saint Paul, he was later arrested and died on the cross. The "successor" refers to all the successive Popes, since Peter is sometimes regarded as the first of that office.
Based upon this line, commentator G. Ferretti has concluded that the "Comedy" was composed in two periods - he believes that Dante's opinions on the papal office as expressed in the poem contradicts his earlier works "De Monarchia" and "Vita Nova" with their pope-friendly Guelfism, where he expresses opinions he would later revise. Sapegno concurs, seeing an element of doubt and wavering in Dante's political thought during the first period. Reggio, however, disagrees and cites Book 4 of the Convivio and the first Cantos of the Comedy as examples of the maturity and stability of Dante's thought.