I am having a discussion with my friend about what church doctrine is applied to the 6th Commandment, "Thou shall not commit adultery." I believe it falls under the doctrine of faith/salvation, he believes it falls under the doctrine of marriage. I argue that the last five commandments of "...shall nots" really falls under the umbrella of soteriology and that to single #6 out for marriage would not fit the context of these 5 commandments. Does the Catholic Church have a preference or position on this?
What doctrine of the Catholic Church covers the commandment, "Thou shall not commit adultery."?
The Ten Commandments or the Decalogue engulfs the Catholic Doctrine on sin and the moral law.
The Catholic Church teaches that moral law comes from God as “fatherly instructions” to help us determine what is right and what is wrong. It prescribes for man the ways, the rules of conduct that lead to the promised beatitude; it proscribes the ways of evil which turn him away from God and his love. It is at once firm in its precepts and, in its promises, worthy of love.
Catechism of the Catholic Church 1950
Moral law is the means that God keeps us and directs us toward heaven and away from evil. Moral law should be the basis for civil laws in order to draw all people closer to God.
The natural moral law is the knowledge of right from wrong that all people possess as part of human nature, regardless of culture or religion. For example, there is a universal consensus that murder is wrong. That is a natural law.
The natural law is written and engraved in the soul of each and every man, because it is human reason ordaining him to do good and forbidding him to sin . . .
The natural law should be the basis for all civil laws in order to bind people together and create a just society.
It also provides the indispensable moral foundation for building the human community.
The Old Testament
In Old Testament law (old law) God prepared us for the coming of Jesus. The Ten Commandments revealed basic truths which are part of natural law in a way that people could understand and follow. They taught that we must love God and neighbour.
The Old Law is the first stage of revealed Law. Its moral prescriptions are summed up in the Ten Commandments. The precepts of the Decalogue lay the foundations for the vocation of man fashioned in the image of God; they prohibit what is contrary to the love of God and neighbor and prescribe what is essential to it.
The sins against the sixth commandment are multiple. Here are a few listed in Wikipedia’s page on the Ten Commandments in Catholic theology
"You shall not commit adultery."
The sixth commandment according to the Catechism of the Catholic Church.
Offences against chastity
The Catechism lists the following "offenses against chastity", in increasing order of gravity according to Kreeft:
Lust: the Church teaches that sexual pleasure is good and created by God, who meant for spouses to "experience pleasure and enjoyment of body and spirit". Kreeft says, "Lust does not mean sexual pleasure as such, nor the delight in it, nor the desire for it in its right context." Lust is the desire for sexual pleasure alone, outside its intended purpose of procreation and the uniting of man and woman, body and soul, in mutual self-donation.
Masturbation is considered sinful for the same reasons as lust, but is a step above lust in that it involves a physical act instead of a mental one.
Fornication is the sexual union of an unmarried man and an unmarried woman. This is considered contrary to "the dignity of persons and of human sexuality" because it is not ordered to the "good of spouses" or the "generation and education of children."
Pornography ranks higher because it is considered a perversion of the sexual act that is intended for distribution to third parties for viewing.
Prostitution is considered sinful for both the prostitute and the customer; it reduces a person to an instrument of sexual pleasure, violating human dignity and harming society. The gravity of the sinfulness is less for prostitutes who are forced into the act by destitution, blackmail or social pressure.
Rape is an intrinsically evil act that can cause grave damage to the victim for life.
Incest, or "rape of children by parents or other adult relatives" or "those responsible for the education of the children entrusted to them" is considered the most heinous of sexual sins.
Offences against the dignity of marriage
According to the Church, adultery and divorce are considered offenses against the dignity of marriage and are defined as follows:
Adultery is the sexual union of a man and woman where at least one is married to someone else. It is for this reason that the Church considers it a greater sin than fornication. Kreeft states, "The adulterer sins against his spouse, his society, and his children as well as his own body and soul."
Divorce: According to the Catholic New American Bible translation, Jesus taught, "whoever divorces his wife (unless the marriage is unlawful) causes her to commit adultery, and whoever marries a divorced woman commits adultery." Explaining Church interpretation of this teaching, Kreeft says Jesus considered divorce to be an accommodation that had slipped into Jewish law. The Church teaches that marriage was created by God and was meant to be indissoluble: like the creation of a child that cannot be "un-created", neither can the "one flesh" of the marriage bond. The Catechism states, "Divorce is a grave offense against the natural law. It claims to break the contract, to which the spouses freely consented, to live with each other till death." By marrying another, the divorced person adds to the gravity of the offense as the remarried spouse is considered to be in a state of "public and permanent adultery".
The Compendium of the Catechism 502 lists other offenses against the dignity of marriage: "polygamy, incest, free unions (cohabitation, concubinage), and sexual acts before or outside of marriage".