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

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

(Youcat answer) 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 1852) There are a great many kinds of sins. Scripture provides several lists of them. The Letter to the Galatians contrasts the works of the flesh with the fruit of the Spirit: "Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, factions, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the Kingdom of God" (Gal 5:19-21; cf. Rom 1:28-32; 1 Cor 9-10; Eph 5:3-5; Col 3:5-8; 1 Tim 9-10; 2 Tim 2-5).

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 1853) Sins can be distinguished according to their objects, as can every human act; or according to the virtues they oppose, by excess or defect; or according to the commandments they violate. They can also be classed according to whether they concern God, neighbor, or oneself; they can be divided into spiritual and carnal sins, or again as sins in thought, word, deed, or omission. The root of sin is in the heart of man, in his free will, according to the teaching of the Lord: "For out of the heart come evil thoughts, murder, adultery, fornication, theft, false witness, slander. These are what defile a man" (Mt 15:19-20). But in the heart also resides charity, the source of the good and pure works, which sin wounds.   

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