We are all of us born ignorant of just about everything, and even if we live diligently we will still have a lot to learn by the time we die. The natural consequence of this inevitable state of ignorance is that we will constantly make mistakes due to our lack of knowledge of the truth. In fact, it might be legitimate to say that man in general, insofar as he is fallen, is more often separated from God due to ignorance than to evil plain and simple, and that the sins a man commits are more often the result of wrong-headedness than hard-heartedness. According to Pope Leo XIII:
“It is rather ignorance than ill-will which keeps multitudes away from Jesus Christ. There are many who study humanity and the natural world; few who study the Son of God. The first step, then, is to substitute knowledge for ignorance, so that He may no longer be despised or rejected because He is unknown.”
Observing this, and considering the fact that a man cannot be held responsible for a sin in which his will gave no real assent, we can say that this inevitable, invincible sort of ignorance is not a sin. However, there is an underlying assumption that goes along with this notion of invincible ignorance, which is that we have at the same time done our best to overcome it and minimize its impact on our lives.
 TFP, 13.