I am really confused about mortal sins. Is there a whole list of mortal sins a person can commit?

I’ve never seen an exhaustive list of mortal sins myself, but sometimes pamphlets with examinations of conscience (for going to Confession) will list out some sins. The Fathers of Mercy have one on their website if you search for it.

There are two kinds of sins: mortal sin and venial sin. All sin is wrong and damaging to the soul, but whereas venial sin makes the soul “sick,” mortal sin is deadly (which is where the name comes from). Venial sin is like spiritual petty crime or misdemeanor; mortal sin is like a spiritual felony–except even worse! Venial sin wounds our relationship with God, but when people commit mortal sin, they are cutting themselves off from relationship with God. It’s a radical misuse of human freedom, contrary to love. The end of mortal sin is spiritual death–that is, hell.

As for what makes a mortal sin, the Catechism discusses this in sections 1854-1864. Section 1857 lists the conditions specifically, and there are three:

1. Grave matter,
2. Full knowledge,
3. And full consent.

A grave matter is basically a “serious” matter. All sin is serious, of course, but some cases are much more serious than others. For instance, murder is a serious matter. Adultery is a serious matter. Idolatry is a serious matter. These are actions which seriously offend God and/or seriously violate the dignity of a human person.

Full knowledge means that the person knows that the thing he/she is doing is a grave sin. For example, intentionally skipping Mass on Sunday is actually a mortal sin. However, if someone doesn’t know it’s a mortal sin, then it wouldn’t be a mortal sin. You can’t be held responsible if you’re ignorant! But since I just told you it’s a mortal sin, I guess you can’t use that excuse any more, right? 🙂

Anyway, full consent means that the person intentionally chooses to commit the sin. Let’s take the Mass example. You’re going to an evening Mass and it’s the last Mass of the day (so you won’t have another chance to go). You leave for Mass at the normal time, but then you’re unexpectedly caught in traffic because there was a car accident on the way, and you totally miss Mass. You didn’t mean to miss Mass, because you had no idea there was a car accident. You wouldn’t be committing a sin in this situation, because you didn’t consent to miss Mass. Another popular example is murder. Intentionally killing another human being is always a mortal sin. But what if you’re out driving, a person runs in front of your car, and that person dies? You wouldn’t be committing a mortal sin in that case, because you didn’t make an intentional choice.

If any of those three conditions are missing, then the action can still be a sin, but it would not be a mortal sin.

If you’re having doubts or concerns about whether or not a particular action is a mortal sin or not, the Catechism is a great resource!