I fully understand that mortal sin requires grave matter, sufficient consent, and sufficient knowledge. But how do you know when some action was grave?
I know the basic examples, like how murder is grave but glaring is not, and adultery is grave but doing a double-take at someone while walking down the road is not. But I can’t take an educated guess in most other instances.
Some examples from the Catechism. Every commandment involves grave matter.
2117 All practices of *magic *or sorcery, by which one attempts to tame occult powers, so as to place them at one’s service and have a supernatural power over others - even if this were for the sake of restoring their health - are gravely contrary to the virtue of religion
2120 *Sacrilege *consists in profaning or treating unworthily the sacraments and other liturgical actions, as well as persons, things, or places consecrated to God. Sacrilege is a grave sin …
2148 … Blasphemy is contrary to the respect due God and his holy name. It is in itself a grave sin.79
**2152 **… Perjury is a grave lack of respect for the Lord of all speech. …
2181 The Sunday Eucharist is the foundation and confirmation of all Christian practice. For this reason the faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor.119 Those who deliberately fail in this obligation commit a grave sin.
2355 *Prostitution *does injury to the dignity of the person who engages in it, reducing the person to an instrument of sexual pleasure. The one who pays sins gravely against himself: he violates the chastity to which his Baptism pledged him and defiles his body, the temple of the Holy Spirit.140 Prostitution is a social scourge. It usually involves women, but also men, children, and adolescents (The latter two cases involve the added sin of scandal.). While it is always gravely sinful to engage in prostitution, the imputability of the offense can be attenuated by destitution, blackmail, or social pressure.
2356 *Rape *is the forcible violation of the sexual intimacy of another person. It does injury to justice and charity. Rape deeply wounds the respect, freedom, and physical and moral integrity to which every person has a right. It causes grave damage that can mark the victim for life. It is always an intrinsically evil act. Graver still is the rape of children committed by parents (incest) or those responsible for the education of the children entrusted to them.
2376 Techniques that entail the dissociation of husband and wife, by the intrusion of a person other than the couple (donation of sperm or ovum, surrogate uterus), are gravely immoral. …
2384 *Divorce *is a grave offense against the natural law. …
2396 Among the sins gravely contrary to chastity are masturbation, fornication, pornography, and homosexual practices.
2413Games of chance (card games, etc.) or *wagers *are not in themselves contrary to justice. They become morally unacceptable when they deprive someone of what is necessary to provide for his needs and those of others. The passion for gambling risks becoming an enslavement. Unfair wagers and cheating at games constitute grave matter, unless the damage inflicted is so slight that the one who suffers it cannot reasonably consider it significant.
2434 A* just wage* is the legitimate fruit of work. To refuse or withhold it can be a grave injustice. …
2539 Envy is a capital sin. It refers to the sadness at the sight of another’s goods and the immoderate desire to acquire them for oneself, even unjustly. When it wishes grave harm to a neighbor it is a mortal sin: …
2480 Every word or attitude is forbidden which by flattery, adulation, or complaisance encourages and confirms another in malicious acts and perverse conduct. Adulation is a grave fault if it makes one an accomplice in another’s vices or grave sins.
2484 The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity.
2491Professional secrets - for example, those of political office holders, soldiers, physicians, and lawyers - or confidential information given under the seal of secrecy must be kept, save in exceptional cases where keeping the secret is bound to cause very grave harm to the one who confided it, to the one who received it or to a third party, and where the very grave harm can be avoided only by divulging the truth.
tommydude2112 and others, note the followiing from Catholic Answers, it is not necessary to know that the sin is grave for it to be a mortal sin.
" …one does not have to know with metaphysical certitude that a given act is gravely sinful. Lesser degrees of knowledge will also count—again, the degree of knowledge needed for an authentically human act is the key. Indeed, someone might even be feigning ignorance or being hardhearted toward the evidence in such a way that he is responsible for knowing something even though he professes not to know it."