Commentary Par XXXII 136-138

The 'greatest father' is, of course, Adam. This is Saint Lucy's third appearance in the poem (see [Inf II 97] and [Purg IX 52-63]).  Bernard here reminds Dante of the first one, when he was 'ruining' downward back toward death ([Inf I 61]) when Virgil appeared to him, the result of the collaboration of the Virgin, Lucy, and Beatrice (seated at first where she is right now, next to Rachel [[Inf II 102]]).