Youcat commented through CCC – Question n. 316 - Part II.

YOUCAT Question n. 316 - Part II. How can we distinguish serious sins (mortal sins) from less serious (venial) sins?

(Youcat answer - repeated)  Serious sin destroys the divine power of love in a person’s heart, without which there can be no eternal beatitude. Hence it is also called mortal sin. Serious sin breaks with God, whereas venial sin only strains the relationship with him.

A deepening through CCC

(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.     

Reflecting and meditating 

(Youcat comment) A serious sin cuts a person off from God. One requirement for such a sin is that it be opposed to an important value, for instance, directed against life or God (for example, murder, blasphemy, adultery, and so on) and that it be committed with full knowledge and full consent. Venial sins are opposed to secondary values (honor, truth, property, and so on) or are committed without full knowledge of their seriousness or without full consent of the will. Such sins disrupt the relationship with God but do not sever it.

(CCC Comment) (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.).

 
(This question: How can we distinguish serious sins (mortal sins) from less serious (venial) sins? is continued)