To sin through ignorance is not a sin; that is part of the general evil in the world as it is, but to sin when you know, that is serious. It means that there is hidden somewhere, like a worm in the fruit, an element of bad will that must be hunted out and destroyed, at any cost, because any weakness on such a point is the source of difficulties that sometimes, later on, become irreparable.
So then the first thing is to be perfectly happy when someone or some circumstance puts you in the conscious presence of a fault in yourself which you did not know. Instead of lamenting, you must rejoice and in this joy must find the strength to get rid of the thing which should not be.
Ref : Questions and Answers (1929 – 1931)