Thank you for the question on fornication in the Old and New Testament, “Why is fornication not mentioned in the Old Testament of the Bible but only in the New Testament (in Apostle Paul’s Epistles)?”
The definition of fornication is sexual intercourse between people who are not married to each other. The emphasis is on individuals who are not married to each other. Thus it is any extra marital sexual relationship.
For one’s consideration of the prohibition of “fornication” in the Old Testament:
A. In the Ten Commandments, the Sixth Commandment prohibits adultery.
The Sixth Commandment is this: “You shall not commit adultery.” Adultery is understood as sexual intercourse between a married person to another married person who is not one’s spouse or to someone who is unmarried. There is the bond of oneness between the husband and wife that is to be sacred, forsaking all others (Gen. 2:23–24).
The term, immorality, is a broader term that would include adultery as well as any deviate conduct as homosexuality or bestiality (sex with animals).
B. In Leviticus 18, the Sixth Commandment is expounded upon to explain what is prohibited by God.
I cite for example:
4 You must obey my laws and be careful to follow my decrees. I am the Lord your God. 5 Keep my decrees and laws, for the person who obeys them will live by them. I am the LORD.
6 “‘No one is to approach any close relative to have sexual relations. I am the LORD. 7 “‘Do not dishonor your father by having sexual relations with your mother. She is your mother; do not have relations with her. 8 “‘Do not have sexual relations with your father’s wife; that would dishonor your father. 9 “‘Do not have sexual relations with your sister, either your father’s daughter or your mother’s daughter, whether she was born in the same home or elsewhere. 10 “‘Do not have sexual relations with your son’s daughter or your daughter’s daughter; that would dishonor you. 11 “‘Do not have sexual relations with the daughter of your father’s wife, born to your father; she is your sister….Do not have sexual relations with a man as one does with a woman; that is detestable. 23 “‘Do not have sexual relations with an animal and defile yourself with it. A woman must not present herself to an animal to have sexual relations with it; that is a perversion” (NIV).
The Apostle Paul was a Pharisee (Phil. 3:4b-6). He would have known the Torah or the Law of Moses forward and backward. In his writings to Gentile believers was to call them to live godly lives, not ungodly lives. Thus in one sense one can say that the Old Testament is more specific than the New Testament of immoral conduct.
The prohibitions listed in the Old Testament, in my opinion, is not exhaustive, but providing the principles for priests to judge a case that violates the Law of Moses.
SUMMARY: The word, fornication, in the New Testament is a near equivalent to word, adultery, in the Old Testament.
For more perspectives: