This is the tale of Cecilia, so even though there are many characters in "The Second Nun's Tale" who demonstrate great principles (or holiness), it's Cecilia's that gets described and commented on the most. Cecilia's chastity is one of the most important aspects of her holiness, for it's through this chastity that she's able to make converts to the Christian faith. These converts are the spiritual children that replace the physical ones the chaste Cecilia will never have, and they are the tangible signs of her holiness. Another aspect of this holiness is Cecilia's preaching and teaching. The fact that she continues this preaching and teaching even as she's dying indicates that, for Cecilia, this aspect of her holiness is her most important mission on earth.
"The Second Nun's Tale" portrays Cecilia's ability to make converts as the most important aspect of her holiness.
"The Second Nun's Tale" implies that true vision is possible only when one becomes free of sin.