Fornication and the Ten Commandments: What "Thou Shalt Not Commit Adultery" Covers

Does the Ten Commandments list mention fornication?

No: the word "fornication" does not appear in the Ten Commandments, which forbid adultery but say nothing explicit about sex between unmarried people. In the King James Version, Exodus 20:14 reads simply "Thou shalt not commit adultery," between "Thou shalt not kill" and "Thou shalt not steal" ^[1].

The commandments are also known as the Decalogue, from the Greek deka logoi, "ten words." According to Britannica, tradition holds that they were revealed to Moses on Mount Sinai and engraved on two tablets of stone ^[3]. The list appears twice in the Bible, in Exodus 20 and in Deuteronomy 5, where the KJV renders the same commandment "Neither shalt thou commit adultery" ^[8].

Only one other commandment touches on sexual relationships. The last verse of the Exodus list forbids coveting a neighbour's house, wife, servants, ox or ass ^[1]. The two versions differ slightly here: Exodus 20:17 names the neighbour's house before his wife, while Deuteronomy 5:21 puts the wife first and uses a separate verb, "desire," for her ^{[1] [8]}. That difference matters for how the commandments are counted, as explained below. Neither text names premarital sex, so any link between the Decalogue and fornication comes from interpretation: either a broad reading of "adultery" or a reading of the commandments as headings for wider areas of moral life.

How three traditions number the Ten Commandments (Exodus 20)

Commandment (KJV wording, shortened)	Jewish	Catholic and Lutheran	Orthodox and Reformed
I am the LORD thy God	1st	Introduction	Introduction
Thou shalt have no other gods before me	2nd	1st	1st
Thou shalt not make any graven image	2nd (with the above)	1st (with the above)	2nd
Thou shalt not take the name of the LORD in vain	3rd	2nd	3rd
Remember the sabbath day	4th	3rd	4th
Honour thy father and thy mother	5th	4th	5th
Thou shalt not kill	6th	5th	6th
Thou shalt not commit adultery	7th	6th	7th
Thou shalt not steal	8th	7th	8th
Thou shalt not bear false witness	9th	8th	9th
Thou shalt not covet (wife, house, goods)	10th	9th and 10th (split in two)	10th

What did adultery mean in Exodus 20?

In the law of ancient Israel, adultery meant sexual intercourse between a married woman and a man other than her husband. The Jewish Encyclopedia, in an article by David Werner Amram published in its 1906 edition, states that the crime could be committed "only by and with a married woman" ^[6]. Intercourse between a married man and an unmarried woman was therefore not technically adultery in Jewish law ^[6].

Unlawful intercourse with a betrothed woman, promised in marriage but not yet wed, was also treated as adultery ^[6]. The penalty was severe: under biblical law both the guilty wife and her partner were put to death, citing Deuteronomy 22:22 ^[6]. The encyclopedia adds that the Mishnah prescribed strangulation, and that the death penalty was abolished around the year 40, after which adulterers were scourged ^[6].

Sex between an unmarried man and an unmarried woman was handled elsewhere in the law, and in a different register. Exodus 22:16 provides that a man who entices a maid "that is not betrothed" and lies with her must endow her to be his wife; if her father refuses, the man pays the dowry of virgins instead ^[7]. The text treats the act as a matter of marriage and compensation for the family, not as a capital offense, which shows how far its categories were from the modern word "fornication." The Catholic Catechism observes that the first three commandments concern love of God and the other seven love of neighbour ^[4], which places adultery among the duties owed to other people.

Why is it the sixth commandment for some and the seventh for others?

The adultery commandment carries different numbers because traditions divide the same text into ten in different ways. The Catechism of the Catholic Church acknowledges that the division and numbering have varied in the course of history and says it follows the division established by St. Augustine, which is also that of the Lutheran confessions ^[4]. The Greek Fathers worked out a slightly different division, found in the Orthodox Churches and Reformed communities ^[4].

In the Augustinian scheme, the ban on other gods and the ban on images form a single first commandment, and the prohibition on coveting is split in two, so adultery becomes the sixth ^[5]. The Orthodox and Reformed scheme separates other gods from images and keeps coveting as one, which makes adultery the seventh ^[5]. Jewish tradition counts "I am the LORD thy God" as the first word, groups other gods and images as the second, and also arrives at adultery as the seventh ^[5].

The Catholic Catechism therefore discusses sexual ethics in its article on "the sixth commandment" ^[2], while Presbyterian standards discuss the same subject under the seventh ^[9]. Catholic apologist James Akin, writing in 1996, notes that the Catholic Church has made no formal determination on the matter and that Catholics are free to prefer another division ^[5]. The text is the same in every case; only the counting differs.

How do churches extend the commandment to sex before marriage?

Most Christian traditions read the adultery commandment as the heading for all of sexual ethics, which brings fornication within its scope. The Catholic Catechism states this principle plainly: "The tradition of the Church has understood the sixth commandment as encompassing the whole of human sexuality" ^[2].

The same article draws on the Sermon on the Mount, where Jesus extends the commandment from the act of adultery to looking at a woman lustfully (Matthew 5:27-28) ^[2]. Within its commentary on the sixth commandment, in the section on the vocation to chastity, the Catechism defines fornication as carnal union between an unmarried man and an unmarried woman and calls it gravely contrary to the dignity of persons ^[10]. Adultery, defined as marital infidelity, is treated in a separate section of the same article ^[12]. More on Catholic teaching is in What the Catholic Catechism Says About Fornication.

Reformed Protestants reach the same result through the seventh commandment. The Westminster Larger Catechism, a seventeenth-century Reformed text still published by the Orthodox Presbyterian Church among its standards, answers that the seventh commandment is "Thou shalt not commit adultery" (question 137) ^[9]. Among the duties it requires, it lists chastity in body, mind, affections, words and behavior, and marriage for those who lack the gift of continence (question 138) ^[9]. The same answer names "conjugal love, and cohabitation" as duties of spouses ^[9], a reminder that the word, in this text, refers to husband and wife living together rather than to an unmarried couple sharing a home, the sense it has taken on in modern American usage.

Question 139, on the sins the commandment forbids, names adultery, fornication, rape and incest in its first line ^[9]. The same answer also condemns the prohibition of lawful marriages, entangling vows of single life and the undue delay of marriage ^[9]. The commandment thus becomes a full program for sexual life, extending in both directions: against sex outside marriage and against obstacles to marriage.

Where does the Bible address fornication directly?

The Bible addresses sex outside marriage directly in other passages, mainly in the New Testament through the Greek word porneia. Blue Letter Bible's lexicon counts 26 occurrences of porneia in the Textus Receptus, all rendered "fornication" in the King James Version ^[11].

Some of those passages show that New Testament writers themselves kept the two ideas apart. Thayer's lexicon, quoted on the same page, notes that porneia is distinguished from moicheia, the Greek word for adultery, in Matthew 15:19, Mark 7:21 and Galatians 5:19, where both appear in the same list ^[11]. In other places porneia has a wider sense: the lexicon's definition covers illicit intercourse in general, including adultery and incest, and figuratively idolatry ^[11].

Among the best-known verses are Paul's command in 1 Corinthians 6:18, examined in "Flee Fornication": Reading 1 Corinthians 6:18, and his advice in 1 Corinthians 7:2 to marry "to avoid fornication" ^[11]. The term also appears in Acts 15:20 and 15:29 and repeatedly in the Book of Revelation ^[11].

In the Hebrew Bible, the clearest text on intercourse between unmarried partners is the law of Exodus 22:16-17, which requires the man to marry the young woman or pay the dowry of virgins ^[7]. The law speaks of marriage and the father's consent rather than of sin, a framework quite different from later Christian teaching on fornication, which treats the act itself as the problem.

What changes when the commandment is read narrowly or broadly?

The two readings lead to different answers about what the Ten Commandments themselves require, even when the overall moral teaching is similar. On a narrow, historical reading, close to the Jewish legal definition, the seventh commandment protects marriage, and premarital sex is governed by other laws ^{[6] [7]}.

On the broad reading shared by Catholic and Reformed catechisms, the commandment is the heading under which all sexual conduct is examined, so fornication breaks it even though the text does not name it ^{[2] [9]}. The broad reading also allows these catechisms to include inner attitudes, such as lustful thoughts, and positive duties, such as chastity and marital love.

For believers the practical result is often the same, since churches that use the broad reading also cite New Testament texts on porneia. The difference matters more in debates about the Bible: readers who ask what the Decalogue literally says find adultery, while readers who ask what the commandment has meant in Christian teaching find a rule covering every form of sex outside marriage. The site's language section compares the two words in Fornication or Adultery?.

Frequently asked questions

Which commandment is about sex?

The commandment "Thou shalt not commit adultery" (Exodus 20:14) is the one about sexual conduct ^[1]. It is the sixth commandment for Catholics and Lutherans and the seventh for Jews, Orthodox and Reformed Christians ^{[4] [5]}. The prohibition on coveting a neighbour's wife also concerns sexual desire ^[1].

Is fornication worse than adultery in the Bible?

No. In biblical law adultery was the graver offense, punishable by death for both partners according to Deuteronomy 22:22 ^[6], while sex with an unbetrothed young woman led to marriage or a payment to her father under Exodus 22:16-17 ^[7]. The New Testament condemns both and lists them side by side ^[11].

Does the tenth commandment relate to sexual desire?

Yes, in part. In the Jewish, Orthodox and Reformed numbering, the tenth commandment forbids coveting a neighbour's wife along with his house and goods ^{[1] [5]}. Catholic and Lutheran catechisms split coveting into two commandments, the ninth and tenth, so that desire for another's spouse is treated on its own ^[5].

Do Jews number the Ten Commandments differently?

Yes. Jewish tradition counts "I am the LORD thy God" as the first word and combines the bans on other gods and on images into the second ^[5]. Adultery is therefore the seventh, as in Orthodox and Reformed churches, but not the sixth as in the Catholic numbering ^[5].

Is "Thou shalt not commit adultery" also in Deuteronomy?

Yes. Deuteronomy 5 repeats the Ten Commandments, and the KJV renders verse 18 as "Neither shalt thou commit adultery" ^[8]. In that version the prohibition on desiring a neighbour's wife comes first in the final verse, before the neighbour's house and field ^[8].

Did Jesus change the meaning of the adultery commandment?

Christian tradition holds that he deepened it. In Matthew 5:27-28, quoted by the Catholic Catechism, Jesus says that looking at a woman lustfully is already adultery in the heart ^[2]. Churches cite this passage to read the commandment as covering desires and all sexual conduct, not only the act of adultery.