Where to begin? The sense of vision and sight is such an important symbol in "The Second Nun's Tale," it's hard to connect it to just one thing – so we'll connect it to several. First of all, characters can only see certain supernatural things – like Cecilia's angel, or the crowns of roses and lilies – after they've accepted the truths of Christianity. Furthermore, Cecilia remarks that even though there's nothing whatsoever wrong with Almachius's eyes, he can't 'see' that his gods are really made of stone.
So what's up with vision here? Well, in "The Second Nun's Tale," the heightened vision characters receive upon their conversion to Christianity might represent the heightened spiritual gifts Christianity offers its adherents. Furthermore, a character like Almachius's inability to perceive certain physical truths through sight – like the fact that his idols are stone – represents his lack of spiritual acuity. So to sum up, in "The Second Nun's Tale," the sense of sight symbolizes the heightened perception of spiritual truths available to Christian converts.