Our tendency to commit Actual Sin originated with the Fall, via the concupiscence we acquire from Original Sin. Actual Sin is divided into mortal and venial sin. Mortal Sin “destroys charity in the heart of man by a grave violation of God’s law” (CCC 1855). Those who do not repent of mortal sin before they die cannot go to Heaven (CCC 1033).
For a sin to be mortal, three conditions must be met. 1.) the sin must deal with grave matter, 2.) the sinner must commit the sin with full knowledge of it, 3.) the sinner must give full consent.
The Catechism of the Catholic Church defines grave matter as a serious violation of the ten commandments. Likewise, the gravity of the sin depends on who is offended; “violence against parents is in itself greater than violence against a stranger” (CCC 1858). There is not an exhaustive list of all grave matter, nor of mortal sins, but there are certain sins which by their nature constitute grave matter (e.g. murder, adultery, masturbation, perjury, etc.).
Secondly, the sinner must commit the sin with full knowledge of its gravity. This does not mean that he must know that this sin will expose him to hell if he commits it, nor does it mean that he cannot have any reservations about it. Rather, this condition for a mortal sin may be best understood as sufficient knowledge. Sufficient reflection means that one reflects enough on the sin for it to be mortal. Again, this is an unspecific definition, because it is difficult to quantify when a person crosses the threshold for a sin to be mortal. The Catechism best expresses it: a mortal sin “implies a consent sufficiently deliberate to be a personal choice. Feigned ignorance and hardness of heart do not diminish, but rather increase, the voluntary character of a sin” (CCC 1859). If one has made enough of a decision to make a sin a personal choice, and if the other two criteria are fulfilled, he has committed a mortal sin.
Thirdly, the sinner must commit a sin with full consent to be mortal. Full consent does not mean that one must commit the sin while thinking: “I’m committing this sin and I love it!” Rather, the person must have had sufficient reflection on the sin for it to be mortal. If one worries about the sin and whether or not he will give into it, he may be falling into scrupulosity, meaning that he obsesses over sins which are not sins, or thinks venial sins are mortal ones. In such a situation, where one is being tempted and can seemingly find no escape, he should immediately invoke Jesus and Mary by name, saying that he does not want the temptation, and then proceed to think of something else. As long as the attacks continue, he can trust that he does not give in to them. The more he focuses on the temptation, the stronger it will become. Instead, such souls should rest assured that the Lord knows their situation. They should not let temptations hinder them from doing God’s will. However, if someone thinks about the sin while enjoying it and not trying to resist it, then it is mortal, assuming it is grave matter and the sinner has sufficient knowledge of the sin.
If any one of these three conditions is not fulfilled, then the sin is not mortal. There may be venial sin, but there is no mortal sin. One should consult a confessor or trusted spiritual director in these matters, as many of these situations require thought and discretion of a wise and learned person, probably a priest.
Venial sin is a transgression of the moral law in a less serious matter or in a serious matter, when either full consent or full knowledge, or both, are lacking (CCC 1862).