Commentary Par XXXII 11-12

Ruth is identified only by periphrasis and in her role as the great-grandmother of David, also indicated by periphrasis.  He is represented by his sins of adultery and murder (Bathsheba and Uriah), the setting for Psalm 50 (51), Miserere mei (Have mercy on me), which served as the text of Dante's first spoken words as character in the poem ([Inf I 65]).  For the meaning of David for Dante, see C.Purg.X.65 and C.Par.XX.37-39.  And see Carroll's discussion of a common theme behind at least most of Dante's choices (DDP Carroll.Par.XXXII.1-48), which offers another reason for the reference to David: '[T]hey were all regarded as types of the Church, and they are for the most part ancestresses of Christ according to the flesh (Rachel and Judith alone are not in the direct line of our Lord's ancestry.  Judah, through whom the descent flows, was a son of Leah; and Judith had no children [Judith 16:22]): Ruth, for example, is described as the bisava, the great grandmother, of David, for the purpose, apparently, of indicating the descent of the Virgin, and therefore of her Son, from that king.  The manner in which David is referred to -- "the singer who for sorrow of his sin said Miserere Mei" (Ps. 50 [51]:1) -- while apparently irrelevant to the question of descent, is in reality closely connected with it. Matthew 1:6 states plainly that "David the king begat Solomon of her that had been the wife of Urias."  The reference therefore to David's repentance for his great sin, so far from being irrelevant, suggests in the most delicate way the continuation of the descent through Solomon.'