Many people on CAF have asked questions here about whether one of their sins is mortal or not. I haven't seen a single one that met the Vatican's criteria for mortal sin in all my time on CAF. These folks wouldn't ask in the first place if they knew the definition. They'd be able to answer this question for themselves. Knowing what criteria a sin has to meet to be mortal can lend a good deal to a person's peace of mind. People get afraid they're damned over too slight of things.
Now I'm not saying it's not very important to fight venial sin. That is a very important battle everyone on Earth is called to fight. However, second-guessing ourselves all the time with questions over whether or not we've just damned ourselves can cause us to unnecessarily miss Communion and lose our peace, so I want people on these forums to know the mortal sin definition.
So I'm creating this thread, so that people will know it.
The main time people commit mortal sin a good deal are where they are in either habitual mortal sin or addictive mortal sin. Otherwise it's a stupid choice some Catholics might make, but an unusual one for someone who is making an effort in their life to do what the Church teaches. If a Catholic is trying to live faithfully to the Church's teachings and is fighting against venial sin, and is not engaged in some mortal sin addiction, they are very unlikely to commit a mortal sin. So most people here don't have to worry about it. Just keep fighting the venial sins and seeking to live in unity with God and His Church's teachings and you won't end up committing a mortal sin. It's if you let your spiritual life slide that you can become at risk. As Jesus said, "the one who perseveres to the end will be saved."
Here's the Catechism's definition of mortal sin, for anyone unaware :).
1858 Grave matter is specified by the Ten Commandments, corresponding to the answer of Jesus to the rich young man: "Do not kill, Do not commit adultery, Do not steal, Do not bear false witness, Do not defraud, Honor your father and your mother."132 The gravity of sins is more or less great: murder is graver than theft. One must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger.
1859 Mortal sin requires full knowledge and complete consent. It presupposes knowledge of the sinful character of the act, of its opposition to God's law. It also implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart133 do not diminish, but rather increase, the voluntary character of a sin.
1860 Unintentional ignorance can diminish or even remove the imputability of a grave offense. But no one is deemed to be ignorant of the principles of the moral law, which are written in the conscience of every man. The promptings of feelings and passions can also diminish the voluntary and free character of the offense, as can external pressures or pathological disorders. Sin committed through malice, by deliberate choice of evil, is the gravest.
1861 Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God.
The mortal sin has to be deliberate, premeditated (not spur of the moment or accidental), committed with full knowledge of the nature of the sin and of one's transgression of God's Law, and it must be of grave matter -- a very serious sin, not getting impatient with one of your kids after a tough day at work and being unnecessarily sharp with them. We're talking adultery or idolatry here, major sins, not small ones. Such sins are always "grave matter."