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

YOUCAT Question n. 316 - Part IV. 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 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.

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 1861) Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God's forgiveness, it causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. However, although we can judge that an act is in itself a grave offense, we must entrust judgment of persons to the justice and mercy of God. (CCC 1874) To choose deliberately - that is, both knowing it and willing it - something gravely contrary to the divine law and to the ultimate end of man is to commit a mortal sin. This destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death.

(The next question is: How can a person be delivered from a serious sin and reunited with God?)