Commentary Par XVII 31-36

Set off against pagan dark and wayward speech is Christian clarity of word and purpose.  Perhaps Dante refers to Sibylline prophecy that resulted in human sacrifice (see the muffled but telling reference to the killing of Iphigenia in [Inf XX 110-111]).  Such is opposed by a better sacrifice, that of the Lamb, who took on all our sins (see, for the eventual biblical source of the phrase in the liturgy, which pluralizes our sins [peccata], John 1:29: 'Ecce agnus Dei; ecce qui tollit peccatum mundi' [Behold the lamb of God; behold the one who takes away the sins of the world]).