From the Catechism:
**
II. The Definition of Sin
1849 Sin is an offense against reason, truth, and right conscience; it is failure in genuine love for God and neighbor caused by a perverse attachment to certain goods. It wounds the nature of man and injures human solidarity. It has been defined as "an utterance, a deed, or a desire contrary to the eternal law."
121
1850 Sin is an offense against God: "Against you, you alone, have I sinned, and done that which is evil in your sight."
122 Sin sets itself against God’s love for us and turns our hearts away from it. Like the first sin, it is disobedience, a revolt against God through the will to become "like gods,"
123 knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God."
124 In this proud self-exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.
125
Mortal Sin:
**
1857 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."
131
**
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 heart
133 do not diminish, but rather increase, the voluntary character of a sin.
**