For several generations, the Church has used a kind of shorthand in referring to mortal sin, for example, “X is a mortal sin.” The problem is that this general statement is an oversimplification. In order for the individual committing a particular act to be guilty of a mortal sin, three conditions are necessary: grave matter (the act must be intrinsically evil), full knowledge, and deliberate consent (CCC 1857).
It is important to emphasize that even if a particular sinful act does not rise to the level of mortal sin, it is still a sin. No sinful action, even if committed “innocently” will bring a blessing or become good in itself. To sin is always to veer off course and it causes some sort of wound. This is true even if the person is not guilty of committing a mortal sin.
Let’s consider a couple of specific cases of potentially mortal sin and look at the three conditions required to determine that it represents a mortal sin in a particular situation.
Case 1: Skipping Mass on Sunday
Missing Mass on Sunday is a grave matter because we fail to render fitting thanks and praise to God for His goodness. We sin against justice and charity by failing to gather with God’s people at Mass to do so. In addition, at Mass we are instructed by God and fed with the Body and Blood of the Lord. Jesus says, Unless you eat the flesh of the Son of Man and drink his Blood you do not have life within you (Jn 6:53). Therefore, Mass is necessary for us. Skipping Mass is also a direct violation of the Third Commandment and does harm to the First Commandment. Thus, it is grave matter.
Many Catholics today have been poorly instructed and have very few cultural moorings that dispose them to be at Mass each Sunday. Many do not even know that missing Mass is a grave matter. Even if they know that going to Mass is a good thing—surely better than just sleeping in or going shopping—they may not appreciate the seriousness of missing Mass nor understand that the Eucharist is our necessary food. Depending on how responsible they are for this ignorance, their culpability may be reduced, rendering the sin less than mortal.
Read more at Archdiocese of Washington.