Refraining from speaking the truth is also a form of lying. We argue that "white lies" help us to avoid the unpleasant confrontations that would only upset others; but then our whole existence becomes a white lie. And while snow is pure white, in a blizzard it becomes black.
"Lying is a refusal to accept and relate the truth exactly as we see it. And lying does not require effort, as we like to imagine. Lies come effortlessly in the form of feelings, convictions and beliefs. And we refuse to believe that we are so evil to be able to lie effortlessly. Another lie!" - Nietzsche
The priest believes he can never know what is ultimately true because he is not Christ. And he believes that in order to lie the truth must first be known. In this way he immunizes himself against lying! He banishes both truth and falsity from his vocabulary and replaces them with "faith".