Sin is “any word, deed or desire contrary to the Eternal Law.” In this definition, one can see that as contrary to Eternal Law, sin warrants punishment since God is perfectly just. Mortal sin cuts off man from the grace of God since it is an act which is incompatible with grace. Since man is called to an end by nature that he cannot attain by nature, without God’s grace, he cannot attain heaven and, therefore, punishment for mortal sin is eternal (cf. Mt.25:46). Mortal sin is sin in the strict sense of the word since is opposed to God. It can be understood like complete blindness; it cannot be corrected by human actions alone, but requires Divine grace to be restored, which comes from God when sinner’s repent.
Now, venial sins do not cut sinners off completely from God’s grace as mortal sins do, but they impair His grace from working fully in their souls. Both mortal and venial sins have temporal punishment associated with them, but whereas mortal sin warrants everlasting punishment, the punishment for venial sins ends. Since temporal punishment exists with both, even should one repent of their sin and be restored in God’s grace, the effects of their sin would still need to be resolved. For example, should one break his friend’s prized vase out of anger, even after his friend forgives him, the vase will still be broken as that defect in him that would move him to destroy the vase of his dear friend out of anger. Mending these defects can be done on earth through positive acts, or in purgatory if they are not completely resolved in this life. But, only on this earth can acts be mended actively. Therefore, in purgatory, the soul can only satisfy the debt of sin passively or by the positive acts of those still embodied on earth. This is why the Church has always taught the importance of praying for the souls in purgatory.
Punishment can be divided into three types: penal, satisfaction, and medicinal. St. Thomas expresses them well when he says, “Sometimes someone who did not commit the sin may lovingly undergo another’s reconciling penalty, e.g. pay his debts. But in the ordinary sense penalties are imposed for one’s own sins of action and for Original Sin (inasmuch as the abandonment of our human nature to itself stripped of its original integrated state and burdened with all the defects that accompany this stripping was a penalty for Original Sin.) Sometimes, however, men willingly suffer minor impoverishments so as to gain a major enrichment, and then the sufferings are medicinal rather than punitive. As such no particular sin is their cause, unless one says that the very need for medicine is due to our damaged nature and so is a penalty for Original Sin.” (Summa I-II, 87, 7)
Now for a sin to be mortal, it must affect all three powers of the soul: the intellect, the passions, and the will. The sin must be grave matter (which relates to the intellect), willed, and not so influenced by the passions so as to hinder deliberate consent. This is summed up in the Catechism of the Catholic Church when it says, “For a sin to be mortal, three conditions must together be met: ‘Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent.'”
It is important to note that while there are indeed objectively evil acts, one must consider the freedom of the act. For example, antecedent passions would reduce the voluntariness of the act and there for would reduce the culpability. Even though this is the case, one should not counsel anyone that an objectively evil act is “okay.” It must be acknowledged that the act was objectively evil, even if voluntariness was reduced by the passions or something of the like.
Now, for one to be returned to the state of grace after a mortal sin, one must confess both the kind of sin (species) and the number. “The reason all sins must be confessed species and number is because the mercy of Christ must be applied to each action which has precluded the presence of grace.” (Mullady, Lesson 14)
This is not the case for venial sin for venial sin is not sin “in the strict sense of the word.” (Mullady, Lesson 14) With a venial sin, grace still remains since it “does not involve a disorder regarding the end but only a disorder regarding the means to the end.” (Mullady, Lesson 14) When one is absolved of their venial sins either after a full confession of all mortal sins or during the Confiteor, even if they do not confess them in kind or in number. It is, however, highly recommended to confess ones venial sins, not so as to be scrupulous, but to better draw closer to the medicine of Christ’s Mercy. “The more man’s inner freedom is compromised by willing and performing actions which cannot lead to heaven, even if they are not against it, the more lukewarm the individual is about desiring heaven. This is the reason both concern and frequent confession of venial sins is recommended by not required by the Church.” (Mullady, Lesson 14)