Describe symbolism in Doubt by John Patrick Shanley

The reference to Sister Veronica, who is never seen, is nonetheless symbolically important. In Biblical tradition, Saint Veronica was a woman in the crowd watching Christ pass as he carried his cross to his crucifixion who wiped the sweat, blood and tears from his face. She is often held up as an embodiment of mercy and compassion. The irony here is that Sister Veronica, named for this saint, is going blind - meaning that in metaphoric terms, mercy is blind. In other words, there is an inability to see with the eyes of mercy. This can be seen as a reference to Sister Aloysius' merciless persecution of Father Flynn.