Because nuns in Chaucer's time were compelled to read stories of the saints, the tale of Cecilia is an apt selection for the Second Nun simply because she is a nun and is extremely modest and shy. Her invocation to Mary is typical for all stories, but more so here because the story of St. Cecilia is a story of chastity.
The interpretation of a name was a favorite device during Chaucer's day. Although the Second Nun's interpretation is not correct from an etymological viewpoint, it includes a traditional interpretation that identifies Cecilia with the "heavenly lily for her Chaste virginity"; a road for the blind (non-Christian) by the example of her teachings; a combination of heaven and the biblical Leah who represents the active or busy life; and heaven made visible to the ordinary people.
Just as men see the sun, moon, and stars in heaven, so they see in this maiden her faith and magnanimity and also the whole clarity of her wisdom and the excellence of her works. If heaven and Cecilia are the same, then Cecilia represents the medieval Philosopher's Stone, the stone that, in alchemy, could change base metals into precious metals — a metaphor for the purification of the soul. Thus, if heaven and Cecilia are the same, Cecilia can (figuratively and spiritually) change lead into gold or pagans into Christian.
