394. How are sins distinguished according to their gravity?

394. How are sins distinguished according to their gravity? 

(Comp 394) A distinction is made between mortal and venial sin.

“In brief”

(CCC 1873) The root of all sins lies in man's heart. The kinds and the gravity of sins are determined principally by their objects. 

To deepen and explain

(CCC 1854) Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture  (Cf. 1 Jn 16-17), became part of the tradition of the Church. It is corroborated by human experience. (CCC 1855) Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him. Venial sin allows charity to subsist, even though it offends and wounds it. (CCC 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"  (Mk 10:19). 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. 

Reflection

(CCC 1856) Mortal sin, by attacking the vital principle within us - that is, charity - necessitates a new initiative of God's mercy and a conversion of heart which is normally accomplished within the setting of the sacrament of reconciliation: When the will sets itself upon something that is of its nature incompatible with the charity that orients man toward his ultimate end, then the sin is mortal by its very object…  whether it contradicts the love of God, such as blasphemy or perjury, or the love of neighbor, such as homicide or adultery.... But when the sinner's will is set upon something that of its nature involves a disorder, but is not opposed to the love of God and neighbor, such as thoughtless chatter or immoderate laughter and the like, such sins are venial (St. Thomas Aquinas, STh I-II, 88, 2, corp. art.). 

(Next question: When does one commit a mortal sin?)