Saintliness is the noun for the quality of being like a saint: seeming completely good and honest, as if with no faults. We talk about a person's saintliness — the goodness itself, not the person. It is a rather bookish word; in speech you are more likely to hear the adjective saintly. Though actually, people sometimes use saintliness with a small smile, as if nobody is really that perfect. Do not confuse the two forms: a saintly person; her saintliness.