Several practicing Catholics told me that if a person commits a sin but is unaware that it is a sin, it is not a sin for that person, even if they commit murder, or any other sin. How can this concept be reconciled with what our lord said about sin in the gospels?

Answer: The Church has taught at least since the time of Saint Thomas Aquinas in the 13^th century for a sin to be Mortal, three conditions must simultaneously occur: the object of the sin must be grave matter, and it must be committed with full knowledge and deliberate consent (Catechism of the Catholic Church, n. 1857). Since mortal sin requires full knowledge and complete consent, the Catechism goes on to explain, unintentional ignorance can diminish or even remove the imputability of a grave offense, and the promptings of feelings and passions can also diminish the voluntary and free choice character of the offense, as can such things as external pressures or pathological disorders (n. 1860).