The word which has been translated as "fornication" in the New Testament is the Greek word "porneia". However, it is important to remember that Jesus did NOT use Greek when he spoke to his disciples about this subject.

The Greek word used is a translation from the Hebrew words which would have been used by Jesus. These are :-
"taznuth" (noun) - fornication or whoredom.
"zanah" (verb) - to commit fornication or whoredom.

In the primary meaning, these words refer to :-
(a) the sin of a prostitute and the man who consorts with her;
(b) the behaviour of an unmarried woman who "plays the harlot" by engaging in premarital sex.

The word is quite distinct from the Hebrew word for "adultery" (which is "naaph").

The distinction between the two words is shown clearly in Hosea 4:14 where both are used together :-
- daughters (unmarried) "play the harlot".
- brides "commit adultery".

The word for "fornication" is also used in a secondary sense to describe :-
(a) the sin of idolatry (which often included sacred prostitution);
(b) the political alliances made by Israel with other nations from whom they sought protection against enemies instead of relying on their God for protection.

There are a few references where both "fornication" and "adultery" are used in this secondary way to liken Israel to a wife who has left her husband to become a harlot.
e.g. Jeremiah 3:1,2,8 & Ezekiel 16:15,32.

However, this does NOT mean that the words are interchangeable. Rather, it means that in these cases BOTH sins have been committed. Israel is not simply an adulteress who has been unfaithful with another man. She is also a prostitute who has wilfully played the harlot with many lovers. Worse still, she is a harlot of the most wanton sort, for she has actually taken God's gifts to her and used them to hire the favours of her lovers.

When "fornication" and "adultery" are linked in this secondary sense by the prophets, they cannot be used as a ground for rejecting one wife to take another. Rather these cases actually reinforce the permanency of the marriage covenant. The theme of the prophets is repentance and reconciliation and God's faithfulness to his covenant which makes that possible ...... "until death do us part".

Since we must expect that Jesus' teaching on divorce will be in complete harmony with the character of his Father, as revealed in the O.T., we conclude that when Jesus allows "fornication" as a ground for divorce, he does NOT include the secondary use of the word. Where a marriage has been joined by God, the covenant must be kept at any cost. Even in the face of adultery, reconciliation is the goal, not divorce.

This then leads to the further conclusion that the only way in which "fornication" can be understood as a cause of divorce, refers to the primary meaning of the word. When used in this way, as we shall see in chapter 8, "fornication" is a sin which can invalidate a marriage in God's sight.