Sin separates us from God. Sin causes injury to the person committing it. Depending on the nature of the sin, it may harm our neighbor or community.

The Church teaches that there are two kinds of sin, which differ according to their moral gravity.

Mortal sin involves a “grave violation of God’s law.” They are known as “mortal” because if not forgiven, can condemn a soul to eternal separation from God.   There are three parts that must all be present for a sin to be mortal.

1. The subject must be of serious – grave – matter.
2. It must be committed with full knowledge of the sin and the gravity of the offense.
3. It must be done with complete and full consent, so that it is a personal decision to commit the sin, not a compelled or coerced decision.

Paragraph 1858 of the Catechism defines “grave matter” as:

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.” 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.

These next two paragraphs from the Catechism describe other mortal sins:

1866 Vices can be classified according to the virtues they oppose, or also be linked to the capital sins which Christian experience has distinguished, following St. John Cassian and St. Gregory the Great. They are called “capital” because they engender other sins, other vices. They are pride, avarice, envy, wrath, lust, gluttony, and sloth or acedia.

1867 The catechetical tradition also recalls that there are “sins that cry to heaven”: the blood of Abel (killing of the innocent),  the sin of the Sodomites, the cry of the people oppressed in Egypt, the cry of the foreigner, the widow, and the orphan, injustice to the wage earner.

About the consent required for mortal since, the Catechism says at 1860:

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.

And at 1735:

1735 Imputability and responsibility for an action can be diminished or even nullified by ignorance, inadvertence, duress, fear, habit, inordinate attachments, and other psychological or social factors.

That passage is preceded by this discussion of human freedom:

1731 Freedom is the power, rooted in reason and will, to act or not to act, to do this or that, and so to perform deliberate actions on one’s own responsibility. By free will one shapes one’s own life. Human freedom is a force for growth and maturity in truth and goodness; it attains its perfection when directed toward God, our beatitude.

1732 As long as freedom has not bound itself definitively to its ultimate good which is God, there is the possibility of choosing between good and evil, and thus of growing in perfection or of failing and sinning. This freedom characterizes properly human acts. It is the basis of praise or blame, merit or reproach.

1733 The more one does what is good, the freer one becomes. There is no true freedom except in the service of what is good and just. the choice to disobey and do evil is an abuse of freedom and leads to “the slavery of sin.”2

1734 Freedom makes man responsible for his acts to the extent that they are voluntary. Progress in virtue, knowledge of the good, and ascesis enhance the mastery of the will over its acts.