Saintliness is a quality of being so virtuous that you're almost too good to be true. While everyone talks about the saintliness of your dear, departed great-grandmother, you also recall her wicked sense of humor.
In a literal sense, saintliness describes the quality of a saint, someone considered to be holy by a particular religion. This word is also apt for anyone who is extremely good and kind: "I admire the saintliness of that birthday party clown, staying so sweet-natured even when the kids attacked him with balloons!" Other kinds of saintliness are characterized by things like giving most of your money to charity or volunteering on the weekends after a full week of work.