In my prayer book from 1930, it says, in an examination of conscience, under the 6th and 9th Commandment (emphasis mine):

It is sufficient to remind penitents that each and every act, if deliberate, contrary to the holy virtue of Purity--be it in thought or desire, in look, gesture, word, or deed--is a Mortal Sin, and as such must be mentioned in Confession intelligibly, yet modestly

6th and 9th commandment(pg 20)

  1. Thou shalt not commit adultery.
  1. Thou shalt not covet thy neighbor’s wife.

This isn't the first time I've heard this, but this is the first time I can pinpoint to some kind of source.

This specific prayer book, A Manual of Prayers for the Use of the Catholic Laity (pg 280) I should note was "PREPARED AND PUBLISHED BY ORDER OF THE THIRD PLENARY COUNCIL OF BALTIMORE." As is stated in all capitals on the title page. So it is not merely one theologian's opinion that informs the pages.

So, why is it that all sins against the 6th and 9th Commandments are gravely sinful?


3 Answers 3


Prümmer, O.P., Handbook of Moral Theology p. 69:

Three conditions must be verified for mortal sin:

  1. There must be grave matter which is determined by the object and circumstances of the act (or omission) and which is made known to us as such in the first place through the teaching authority of the Church and her theologians. There are some sins which do not admit of slight matter and these are mortal sins "ex toto genere suo" (v.g., lust, blasphemy, etc.); in other sins the matter is not always grave (v.g., in theft, or fasting), and thus the sin may be venial. These are mortal sins "ex genere suo".
  2. Full advertence to the moral nature of the act is required. Therefore where such advertence is defective, sins are always venial (or there may be no sin at all) because of the act's imperfection.
  3. Full consent is also necessary, and this is always presumed to be present where there is full advertence and no external violence. Therefore fear and passion certainly diminish consent but do not destroy it, and they do not prevent mortal sin unless full advertence is lacking.

taken from this answer to "What is a 'human act' in Catholic moral theology?"

Conditions #2 and #3 seem fulfilled in your quote, where it says: "each and every [human] act, if deliberate, [knowingly] contrary to the holy virtue of Purity".

Prümmer, O.P., vol. 1 pp. 247-8 (PDF pp. 285-6) reference on "Materia gravis" says:

“Mortal sins ex toto genera suo ('from their entire genus') do not admit of slight matter” is to be understood thus: any fully voluntary matter of these sins is a grave disorder and hence a grave sin. And yet these sins become venial by the imperfection of the act, when namely full advertence or full consent are lacking. So, e.g., semi-deliberate lascivious thoughts are venial sins, as also are acts secundo-primi of blasphemy.

“Peccata ex toto genera suo mortalia non admittunt parvitatem materiæ” intelligendus est ita: quælibet materia plene voluntaria istorum peccatorum est gravis deordinatio ac proinde grave peccatum. Attamen ista pecca evadunt venialia ex imperfectione actus, quando scil. deficit plena advertentia vel plenus consensus. Sic e. gr. semideliberatæ cogitationes lascivæ sunt peccata venialia, sicut etiam actus secundo-primi blasphemiæ.

Prümmer, O.P., gives the example of furtum (theft) as a sin that is not mortal ex toto genera suo. Theft is only mortal ex genera suo because the theft of something of little value could never be a mortal sin, even if it is stolen with full advertence of the will.

A sin of luxuria (lust) such as fornication always involves grave matter. There are no degrees of fornication*, like there are degrees of value of things that can be stolen. Fornication either occurs or it doesn't.

*Though Alan of Lille (†1203), who thought fornication with a more beautiful woman is less sinful, might disagree; however, he seems to be assessing the sinfulness of fornication on the part of the will, not the matter.

  • This is good. But being that it's quoted from a handbook, it's no different than the book I had. It still does not answer the question of the source of this teaching.
    – user54757
    Commented Mar 24, 2022 at 17:23
  • 1
    @SupportiveDante What source are you looking for? Prümmer, O.P., was one of the 20th century's greatest moral theologians.
    – Geremia
    Commented Mar 24, 2022 at 18:01
  • 2
    I get that. But like any good theologian he's not pulling it out of thin air. He's either drawing a conclusion from the Deposit of Faith, or repeating from the Magisterium. I'm asking for more than just a single theologians word. Why is Prummer saying there are some kinds of sin that are always mortal? Why is lust one of them?
    – user54757
    Commented Mar 24, 2022 at 18:10
  • @SupportiveDante if a kind of sin is always mortal, it's because the matter is always sufficiently grave enough to warrant mortal sin. I guess the question for you would be this: can you think of a sin against that 6th or 9th commandment that is not grave matter? Keep in mind that fantasizing about committing an act which is gravely sinful is also gravely sinful in the Catholic tradition (eg if I fantasize about murdering someone, I am guilty of grave sin, even if I do not act on those fantasies). Fantasizing is distinct from intrusive thoughts which you push away. The latter is not sinful.
    – jaredad7
    Commented May 3, 2022 at 16:36
  • 1
    @SupportiveDante Thanks for pointing out the Prümmer vol. 1 pp. 247-8 (PDF pp. 285-6) reference on "Materia gravis." See what I added to my answer.
    – Geremia
    Commented Jul 23, 2022 at 4:57

According to Catholicism, why are all sins against the 6th and 9th Commandments grave matter ex toto genere suo?

What is the sixth commandment?

Thou shalt not commit adultery.

What is the ninth commandment?

Thou shalt not covet thy neighbor’s wife.

To be crystal clear I would like to start with a classical list of mortal sins every Catholic should know.

The Sixth Commandment, “You shall not commit adultery”

Adultery: Adultery is marital infidelity. A married person who has sexual relations with anyone but their lawful spouse, even transient sexual relations, commits adultery (CCC 2380).

Divorce: The grave sin of divorce condemns those who divorce and remarry (Matthew 5:32) and those who divorce in the civil sense (except by grave dispensation). Hence divorce between two baptized Christians is a mortal sin (CCC 2384).

Fornication: Fornication is carnal union between an unmarried man and an unmarried woman and is a grave sin (CCC 2353). St. Paul condemns fornication in his epistle 1 Corinthians 6:18. All aspects of intimate contact associated with the marriage act also constitute fornication for Jesus said, “I say to you, everyone who looks at a woman with lust has already committed adultery with her in his heart” (Matthew 9:28). If lustful looks are adulterous, how much worse is lustful physical contact?

Pornography: Pornography is the display of intimate real or simulated sexual acts to a third party. Because it removes the marriage act from within the sacramental sanctity of marriage, and perverts sex, it is gravely contrary to charity (CCC 2354). The display of pornography to children and other parties is especially gravely sinful because it is gravely scandalous.

Prostitution: Prostitution reduces a person to an instrument of sexual pleasure and lust. It is gravely contrary to charity and chastity and defiles the body, the temple of the Holy Spirit. However, destitution, blackmail or social pressure can reduce the gravity of the sin. Still, prostitution is always a sin (CCC 2355).

Rape: A person who commits rape violates the respect, freedom, physical and moral integrity of the victim. It is a brutal crime of violence that can physically and psychologically scar a person for life. It is thus a grave sin (CCC 2356).

Homosexual acts: Although it remains to be determined if homosexuality is a genetic, social or personal stigma, homosexual acts are condemned by God and can NEVER be approved by the Church (1 Corinthians 6:9-10, Genesis 19:1-29, Romans 1:24-27 and CCC 2357). If homosexuals are born with the condition, then they are called to live a life of Christian purity and chastity for the greater love of Christ. Such people can experience a life of trial, which all others must treat with compassion and sensitivity.

Incest: ”Incest is intimate relations between relatives or in-laws within a degree that prohibits marriage between them” (CCC 2388). St. Paul condemns incest in his letter to the Corinthians (1 Corinthians 5:1,4-5).

Masturbation: ”Masturbation is the deliberate stimulation of the sexual organs in order to derive sexual pleasure” (CCC 2352). The Church teaches that sex has two main purposes that must be sought in the marriage act: sex is for reproduction of children within a valid marriage, and it is a loving, unifying act between husband and wife. Masturbation violates both aspects of the natural law and is thus a grave sin.

The Ninth Commandment—”You shall not covet…your neighbor’s wife”

Lust: Lust is disordered desire for or inordinate enjoyment of sexual pleasure. It is disordered because sexual pleasure must not be isolated from its true, natural place: within the Sacrament of Matrimony that is ordered to procreation of children and a unifying love between husband and wife (CCC 2351). Lust, a sin and vice of the flesh, is often a difficult vice to overcome. Human weakness of will and lack of conformity to God is a result of the fall of mankind that causes a disorder between soul and body (called concupiscence) which is often manifested in lust. Yet, lust is a sin that can be overcome through prayer and grace through the Christian sacraments. Christ wills that we overcome lust and replace it with Christian love and purity of heart (Matthew 9:28). “Blessed are the pure of heart, for they shall see God” (Matthew 5:8).

Now a word or two on the Latin phrase ex toto genere suo or grave matter!

Grievous Matter

Moral obligations that are binding under pain of mortal sin. The gravity of the matter is determined by the object and circumstance of the action (or omission) and is known in the first place by the teaching authority of the Church, based on divine revelation. Some sins do not admit of slight matter, and these are mortal sins "from their whole nature" (ex toto genere suo), as lust and blasphemy. In other sins the matter is not always grave, as in theft or injustice, and these are mortal "from their nature" (ex genere suo). In every case, however, for a mortal sin there is also required full advertence of the mind to the fact that the matter is serious, and full consent of the will to do or not do what a person knows is a grave command or prohibition.

How to better understand this I will let Peter Kreeft in his How Catholics Live series do it.

Sexual Morality

The Sixth Commandment: You shall not commit adultery

The Ninth Commandment: You shall not covet your neighbor’s wife

Sins against chastity

The Catechism lists six specific sins against chastity: 1) lust, 2) masturbation, 3)fornication, 4) pornography, 5) prostitution, and 6) rape.

  1. “Lust is disordered desire... for sexual pleasure.... [It is] morally disordered when sought for itself, isolated from its procreative and unitive purposes” (cf. 2528; CCC 2351).

Lust does not mean sexual pleasure as such, nor the delight in it, nor the desire for it in its right context. Contrary to what the world thinks, the Church teaches that sexual pleasure is good, not evil. For God invented sex and its pleasure. “‘The Creator...himself established that in the generative function spouses should experience pleasure and enjoyment of body and spirit.... ’”146 (CCC 2362). It is natural and right that great pleasure accompany great things, and the human sexual act is a great thing because of its two great essential purposes: 1) uniting man and woman in “one flesh,” body and soul, in mutual self-donation, and 2) procreating new persons who bear God’s own image and will exist forever – the closest man ever comes to sharing God’s own power of creation.

The essence of sex, like any intelligently designed thing, is in its purpose. Lust, like any sin, must be seen against that background. Lust divorces the two things God designed to be together, it seeks the pleasure apart from the purpose.

No spontaneous thoughts and feelings can be sins until they are willed or consented to by the will. Thoughts and feelings of sexual arousal are not lust; lust is willing the thoughts and feelings just for the pleasure, without the purposes of the marriage union (personal self- donation and procreation).

  1. “By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. ‘Both the Magisterium [teaching authority] of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action.’138 ‘The deliberate use of the sexual faculty, for whatever reason, outside marriage is essentially contrary to its purpose...’”139 (CCC 2352). Masturbation is wrong for the same reason lust is wrong, with the physical act now added to the mental act:

“For here sexual pleasure is sought outside of ‘the sexual relationship...in which...mutual self-giving and human procreation...is achieved’”139 (CCC 2352).

However, “[t]o form an equitable judgment about the subjects’ moral responsibility...one must take into account the affective [emotional] immaturity, force of acquired habit, conditions of anxiety, or other psychological or social factors...” (CCC 2352).

This sin, like lust, is very common and, in that sense, “natural.” But that no more makes it right, or innocent, than the fact that selfishness is common makes it innocent. The “natural law” is not derived from observing how people do in fact usually behave, but from how their human nature is to be fulfilled and respected.

  1. “Fornication is carnal union between an unmarried man and an unmarried woman. It is gravely contrary to the dignity of persons and of human sexuality which is naturally ordered to the good of spouses and the generation and education of children” (CCC 2353). Adultery is even more gravely wrong because at least one of the parties is married to another (see paragraph 16).

  2. “Pornography consists in removing real or simulated sexual acts from the intimacy of the partners in order to display them deliberately to third parties....[I]t perverts the conjugal act, the intimate giving of spouses to each other.... It is a grave offense. Civil authorities should prevent the production and distribution of pornographic materials” (CCC 2354). 5) “Prostitution does injury to the dignity of the person who engages in it, reducing the person to an instrument of sexual pleasure....

  3. 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” (CCC 2355).

  4. “Rape is the forcible violation of the sexual intimacy of another person.... 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” (CCC 2356).

  5. Homosexuality

“Homosexuality refers to [sexual] relations between men or between women who experience...sexual attraction to persons of the same sex. It has taken a great variety of forms through the centuries and in different cultures. Its psychological genesis remains largely unexplained.”

“Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity [Genesis 19:1-29; Romans 1:24-27; 1 Corinthians 6:10; 1 Timothy 1:10],141 tradition has always declared that ‘homosexual acts are intrinsically disordered.’142 They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity [that is, they refuse the divinely designed “otherness” built into sexuality]. Under no circumstances can they be approved” (CCC 2357).

There is no doubt, no “gray area,” and no change in the Church’s teaching about the objective sinfulness of homosexual acts. However, improved psychological and biological knowledge require us to be much less judgmental about the subjective culpability of homosexual persons. “This inclination, which is objectively disordered, constitutes for most of them a trial. They must be accepted with respect, compassion, and sensitivity. Every sign of unjust discrimination in their regard should be avoided” (CCC 2358).

It is also necessary to make a sharp distinction between homosexual desires, or a homosexual “orientation,” and homosexual acts. We are responsible for the acts we choose to perform, but not for the desires we experience (unless we freely will them or consent to them). Homosexual desires are disordered, but they are not sins.

“Homosexual persons are called to chastity” (CCC 2359) just as heterosexual persons are. They need the virtue of self-control to conquer powerfully attractive desires for illicit pleasures, just as heterosexual persons do. And they can be serious and even saintly Christians just as heterosexual persons can.

Birth control

What is usually called “birth control” is really birth prevention. This, the Church opposes. Essentially, the Church’s teaching is 1) that birth is wonderful and 2) that birth control can be legitimate, but 3) that birth prevention (contraception) is not. Each point must be understood in light of the one before it.

  1. “Fecundity is a gift, an end of marriage, for conjugal love naturally tends to be fruitful. A child does not come from outside as something added on to the mutual love of the spouses, [or as an “accident”!] but springs from the very heart of that mutual giving, as its fruit and fulfillment” (CCC 2366).

  2. If two criteria are met, birth control is legitimate: a subjectively good intention and an objectively good means, or method, or way of regulating births. “For just reasons, spouses may wish to space the births of their children. It is their duty to make certain that [a] their desire is not motivated by selfishness but is in conformity with the generosity appropriate to responsible parenthood. [b] Moreover, they should conform their behavior to the objective criteria of morality.... ‘[For] the morality of the behavior does not depend on sincere intention and evaluation of motives alone; but it must be determined by objective criteria, criteria drawn from the nature of the person and his acts...’”156 (CCC 2368). “[M]ethods of birth regulation based on self-observation and the use of infertile periods is in conformity with the objective criteria of morality. These methods respect the bodies of the spouses” (CCC 2370). Natural Family Planning (NFP) is such a method. It is much more reliable than the old “rhythm method,” as successful as “the pill,” and fosters such great intimacy and communication among its users that they have a one percent divorce rate as compared with society’s 50 percent.

“[T]he Church, which ‘is on the side of life’151 teaches that ‘it is necessary that each and every marriage act remain ordered per se to the procreation of human life’”152 (CCC 2366; Humanae Vitae). Man may take advantage of God’s naturally designed infertile periods, but he may not himself try to redesign fertility and lock the door of his fertility against God’s coming. Contraception is “protection” against God.


Adultery is gravely immoral for at least three reasons.

  1. “Adultery is an injustice. He who commits adultery fails in his commitment. He...transgresses the rights of the other spouse...” (CCC 2381).

  2. He does injury to the marriage bond and undermines the institution of marriage.

  3. “He compromises the...welfare of children, who need their parents’ stable union” (CCC 2381). The adulterer sins against his spouse, his society, and his children, as well as his own body and soul.


The Church cannot allow divorce, as almost all Protestant churches do, because she does not have the authority to contradict Christ her Master (see Matthew 5:31-32; 19:3-9; Mark 10:9; Luke 16:18.) “The Lord Jesus insisted on the original intention of the Creator who willed that marriage be indissoluble.174 He abrogates the accommodations [for divorce] that had slipped into the old [Jewish] Law”175 (CCC 2382). In fidelity to her Master, the Church teaches that “‘a ratified and consummated marriage [between two baptized Christians] cannot be dissolved by any human power or any reason other than death’”176 (CCC 2382).

The Church’s prohibition of divorce can be understood only in light of her teaching on marriage. The most important aspect of this teaching, and the one hardest for many today to understand and accept, is that marriage is not a human invention. It has its own unchangeable inner essence, like anything else in nature, as designed by God.

Part of its essence is its indissolubility. Once two people freely create a marriage and become “one flesh,” this cannot be un-created or dissolved “for any reason other than death.” It is like a child. Ending it before death is simply not a possibility offered to us by objective reality.

The ninth Commandment

The ninth Commandment (You shall not covet your neighbor’s wife) adds an internal dimension to the sixth Commandment (You shall not commit adultery), just as the tenth Commandment (You shall not covet your neighbor’s goods) adds an internal dimension to the seventh Commandment (You shall not steal). Already in the Old Testament law God revealed that he wants not only morally good actions but also morally good hearts. For Love is not satisfied with external deeds alone.


“Coveting” your neighbor’s wife or husband is similar to “lusting after” her or him. We are responsible for it, for we choose to do it or not to do it. There is no sin where there is no free choice.

Coveting is to be distinguished from concupiscence, which is not our free choice but our condition (as “original sin” is our condition and each “actual sin” is our choice).

“Concupiscence” means “the movement of the sensitive appetite contrary to the operation of the human reason” (CCC 2515). What reason says No to, concupiscence says Yes to. “Concupiscence stems from the disobedience of the first sin. It unsettles man’s moral faculties and, without being in itself an offense, inclines man to commit sins”303 (CCC 2515). No one can avoid concupiscence. But we can avoid obeying it and being dominated by it. It is like an albatross around our neck, but it need not be our master.

In a nutshell, sins against the 6th and 9th commandments are considered grievous sins in the mind of the Church because God made sex a sacred act that leads to the possibility of human life.

God created man in his own image, in the image of God he created him; male and female he created them. - Genesis 1:27

The creation story in Genesis shows how males and females are intimately connected to each other:

So the Lord God caused a deep sleep to fall upon the man, and while he slept took one of his ribs and closed up its place with flesh; and the rib which the Lord God had taken from the man He made into a woman and brought her to the man. Then the man said, "This at last is bone of my bones and flesh of my flesh; she shall be called Woman, because she was taken out of Man." Therefore a man leaves his father and his mother and cleaves to his wife, and they become one flesh.

This passage from the Bible suggests that man and woman unite to become complete.

St Paul says: Do you not know that your body is a temple of the Holy Spirit within you, which you have from God? You are not your own; you were bought with a price. So glorify God in your body. 1 Corinthians 6:19-20

The Catholic belief is that when a man and woman connect to each other in a sexual way, it is the most intimate physical expression of their total union. For this reason, the Catholic Church teaches in Familiaris consortiothat this union is sacred and is a key element in marriage.

Sexual relationships


It has been the constant teaching of the Roman Catholic Church that sins of impurity are mortal under the standard critera (above).

In regards to these matters, the correct attitude is one of docile obedience to the laws of the Catholic Church. A Catholic accedes to a law, not because one has turned the idea over in his mind and finds it accords with his own reason - no. That is not the virtue of Faith and is a thoroughly Modernist and Protestant error.

The Magisterium is composed of the teaching of the Doctors and Fathers of the Church, whom are each deemed infallible, Papal encyclicals/letters/bulls/public addresses which are also infallible) as well as Sacred Scripture (as interpreted by the Magisterium).

The following approved catechisms cite specific sources. If you are looking for a more comprehensive treatment you would need to go at an approved Moral Theology text used in seminaries before 1958. Or try contacting a well formed priest (such as a priest from Most Holy Trinity Seminary).

Turn a few pages and you will find the corresponding entries for the 9th:




Edit: I suspect that your (and most people's) confusion is with the nature of the obligation of mental assent to the teaching authority of the Church.

The Church's teachings are always based on either the general teaching of theologians, the Fathers, the Doctors and formal Popes or Sacred Scripture. In fact, the Catholic Church is far more scripturally based than any Protestant sect.

This is a comprehensive source on the nature of Catholic Faith vis. a vis. the Popes and the only Church established by Christ: https://novusordowatch.org/the-catholic-papacy/

  • Welcome to Christianity SE and thank you for your contribution. When you get a chance, please take the tour to understand how the site works and how it is different than others.
    – agarza
    Commented Mar 26, 2022 at 12:57
  • Same as what I said to Geremia: this doesn't help with finding a source. It comes close in a footnote on page 198 of your second link, citing Eph. 5:5. But I cannot find a similar interpretation from any Father.
    – user54757
    Commented Mar 26, 2022 at 17:24
  • Aside from how well you answered, you made a factual error: The Doctors are not infallible, and do not themselves constitute a part of the Magisterium. Papal Encyclicals are also not infallible. The Fathers are only infallible in a unanimous interpretation of scripture.
    – user54757
    Commented Mar 26, 2022 at 17:26
  • @SupportiveDante I got that from a well formed Trad source that upon elevation to the status of Doctor, the teaching of the person in question, on Faith and Morals is absorbed into the Magisterium (as opposed to the stricter requirement for theologians (a majority)). Papal Encyclicals, in the context of the Universal Ordinary Magisterium are negatively infallible and demand intellectual assent upon pain of mortal sin. 99% of Traditional Catholics are ignorant of the nature of the UOM. inveritateblog.com/2019/12/15/burke-and-schneider-false-hopes Commented Mar 27, 2022 at 0:04
  • Good answers are supported, meaning that the sources for the claims made in the answer are cited. Thus your answer would have been better had it had the link you just posted.
    – user54757
    Commented Mar 27, 2022 at 0:08

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