When is Sin Mortal?

The Definition of Sin (CCC 1849-1850,1852,1854)

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." (1849)

“Sin is an offense against God.” (1850)  In Psalm 51:4, we read:

Against you, you alone, have I sinned,
    and done what is evil in your sight,
so that you are justified in your sentence
    and blameless when you pass judgment.

And in the Catechism of Catholic Church paragraph 1850 continues:

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," knowing and determining good and evil. Sin is thus "love of oneself even to contempt of God." In this proud self-exaltation, sin is diametrically opposed to the obedience of Jesus, which achieves our salvation.

In paragraph 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. (Gal 5:16-21a)

In paragraph 1854, Sins are rightly evaluated according to their gravity. The distinction between mortal and venial sin, already evident in Scripture, became part of the tradition of the Church.
It is corroborated by human experience.

If any one sees his brother committing what is not a mortal sin, he will ask, and God[a] will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one
is to pray for that. 17 All wrongdoing is sin, but there is sin which is not mortal. (1 Jn 5:16-17)

 
What is Moral Sin?

1. Grave Matter

The action must be of serious nature, violating God's law. This includes actions that are explicitly condemned in the Ten Commandments or other moral teachings. Examples of grave matter
 include murder, adultery, and theft. The seriousness of the act is essential, as it reflects a rejection of God's grace and moral order. 

2. Full Knowledge

The person committing the sin must be fully aware that the act is sinful and contrary to God's law. This means understanding the nature of the act and recognizing its moral implications.
Ignorance or misunderstanding can reduce culpability, but if one knows the act is wrong and still chooses to do it, this condition is met. 

3. Deliberate Consent

The individual must freely choose to commit the act, demonstrating a conscious decision to engage in sinful behavior. This means that the act is not done under coercion or extreme duress, 
and the person has the ability to choose otherwise. If the consent is not deliberate, the sin may not be classified as mortal. 

In summary, for a sin to be classified as mortal, all three conditions must be satisfied. If any one of these conditions is lacking, the sin may be considered venial rather than mortal, 
which does not sever one's relationship with God to the same extent. 

The ‘Eternal Sin’: The True Meaning of ‘Blasphemy Against the Holy Spirit’ is a refusal to repent or seek and trust in God's mercy.  See the article: "Unforgiven: Sins Against the Holy Spirit" on Catholic365

The Church is crystal clear that there are “no limits to the mercy of God.” (CCC 1864) Jesus Christ died for everyone’s sins, and anyone who repents of his sins and follows Christ can receive forgiveness and new life.