The theme of this tale is that God will protect those who are notably virtuous and will punish people who try to abuse the good. This can clearly be seen in the events that occur in Constance's life.
Constance is a complete paragon of virtue. She is a completely good person and everyone who meets her comes to love her unless they are bad people themselves. Because of these traits, God continually protects her when the bad people try to bring about her death.
A secondary theme in the tale is jealousy. The bad people who meet Constance are overwhelmed with jealousy because of her goodness.
These are very Christian themes -- Constance suffers for her goodness but God saves her when this happens.