A common confusion I see among Catholics is a misunderstanding of mortal sin that conflates mortal sin and grave matter. This misunderstanding has serious consequences for that distort the very heart of our faith. Concerning mortal sin, the Catechism says:
“Mortal sin destroys charity in the heart of man by a grave violation of God’s law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him….For a sin to be mortal, three conditions must together be met: “Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.” (CCC 1855, 1857).
Grave matter “is specified by the Ten Commandments” (CCC 1858) and is one of the three necessary components of mortal sin, it itself is not a mortal sin. To borrow an analogy from a friend, say that you have a basic recipe for cookies that uses flour, butter, and sugar. Without any of those ingredients, it’s not a cookie, but that doesn’t mean that whenever one of those ingredients is present that there’s a cookie. Therefore, when someone commits an act of grave evil, they have not necessarily separated themselves from God because grave matter is only one of the three necessary components of a mortal sin.
St. Thomas Aquinas speaks about this as well, he says:
“Nevertheless a sin which is generically mortal, can become venial by reason of the imperfection of the act, because then it does not completely fulfill the conditions of a moral act, since it is not a deliberate….And since a moral act takes its species from deliberate reason, the result is that by such a subtraction the species of the act is destroyed.” (II-I Q.88 Article 6).
In other words, Aquinas wrote that when an action that would otherwise be a mortal sin lacks sufficient freedom, the “species” is destroyed. Brian Killian, in his recent article on conscience, uses the following example to illustrate that point.