Porneia (πορνεια) vs. Moicheia (μοιχεια)

There is a great difference between fornication (porneia [πορνεια]) and adultery (moicheia [μοιχεια]) and a fornicator (pornos [πορνος]) and an adulterer (moichos [μοιχος]). Unfortunately, many false teachers lie and claim that Jesus provided an "exception clause" for divorce with regard to adultery (marital infidelity). If this were so, why then does Jesus (and Matthew's record of it) use the word porneia in this "exception clause" while every other reference to adultery He uses the word moicheia? Because of this glaring problem, these false teachers lie further and claim that the word porneia includes adultery. If this were so, why are there so many passages that use porneia (πορνεια) and moicheia (μοιχεια) side-by-side (Matt. 15:19; Mark 7:21; Gal. 5:19) as well as pornos (πορνος) and moichos (μοιχος) side-by-side (1 Cor. 6:9; Heb. 13:4)? If adultery were included in the word porneia, it would be redundant to add the word moicheia.

With regard to the supposed “exception clause” found in the book of Matthew, further help in discerning what Matthew was saying can be gleaned from the original Greek when we look at the words that he used. This is what John Piper had to say from his study on the subject:

The second thing that began to disturb me was the question, why does Matthew use the Greek word πορνεια (porneia, “sexual immorality”) instead of the word μοιχεια (moicheia) which means adultery? Sexual immorality in marriage would naturally be adultery. But the word Matthew uses to express Jesus’ meaning is one that usually means fornication or sexual immorality without reference to marital unfaithfulness. Almost all commentators seem to make the assumption again that porneia refers to adultery in this context. The question nagged at me why Matthew would not use the word for adultery (moicheia), if that is in fact what he meant.
Then I noticed something very interesting. The only other place besides Matthew 5:32 and 19:9 where Matthew uses the word porneia is in Matthew 15:19 where it is used alongside moicheia. Therefore, the primary contextual evidence for Matthew’s usage is that he conceives of porneia as, in some sense, different than adultery. Could this mean, then, that in Matthew’s record of Jesus’ teaching he is thinking of porneia in its more usual sense of fornication or incest or prostitution that does not denote marital unfaithfulness, that is, adultery?
The next clue in my search for an explanation came when I noticed the use of porneia in John 8:41 where Jewish leaders indirectly accuse Jesus of being born of porneia. In other words, since they don’t accept the virgin birth, they assume that his mother Mary had committed fornication and that Jesus was the result of this act. On the basis of that clue I went back to study Matthew’s record of Jesus’ birth in Matthew 1:18-20.

The word for divorce (απολυσαι) is the same as the word in Matthew 5:32 and 19:9. but most important of all, Matthew says that Joseph was “just” in making the decision to divorce Mary, presumably on account of her assumed porneia, fornication. In other words, this “divorce” was permitted according to Matthew.
Only Matthew has told that story of the crisis Joseph faced in whether to marry his betrothed even though she, as far as he knew at first, had committed fornication (πορνεια). In handling this crisis he called Joseph “just” in the plan to “divorce” her. That means that Matthew, as a follower of Jesus, would not consider this kind of “divorce” wrong. It would not have prevented Joseph (or Mary) from marrying another.
Since only Matthew had told this story and raised this question, he was the only Gospel writer who would feel any need to make clear that Jesus’ absolute prohibition of divorce followed by remarriage did not include a situation like Joseph and Mary’s. That is what I think he does with the exception clauses. He records Jesus saying, “Whoever divorces his wife—not including, of course, the case of fornication [πορνεια] between betrothed couples—and marries another, commits adultery.”
A common objection to this interpretation is that both in Matthew 19:9 and in Matthew 5:31-32 the issue Jesus is responding to is marriage, not betrothal. The point is pressed that “except for fornication” is irrelevant to the context of marriage. My answer is that this irrelevancy is precisely the point of the exception clause. Whether it sounds irrelevant in the context depends on how you hear it. I don’t think it sounds pointless if you hear it the way I just suggested or if Matthew 5:32 goes like this: “But I say to you that everyone who divorces his wife—excluding, of course, the case of fornication [πορνεια] during betrothal—makes her commit adultery.” In this way Jesus makes clear that the action his earthly father almost took—to “divorce” Mary because of πορνεια—would not have been unjust. It would have been right. That is the kind of situation the exception clause is meant to exclude.
This interpretation of the exception clause has several advantages:

• It does not forces Matthew’s Gospel to disagree with the seemingly plain, absolute meaning of Mark and Luke.
• It provides an explanation for why the word porneia is used in Matthew’s exception clause instead of moicheia.
• It squares with Matthew’s own use of porneia (for fornication) in distinction from moicheia (for adultery) in Matthew 15:19.
• It fits Matthew’s wider context concerning Joseph’s contemplated “divorce” from Mary (Matt. 1:19).

John Piper, What Jesus Demands From the World, pp.313-316.

John Piper is correct. If Jesus (and Matthew’s record of it) meant adultery, then the word for adultery—moicheia—would have been used, just as it appears after the so-called “exception” (5:32; 19:9). But it is not. The word used is porneia, which refers to sexual immorality without reference to marital unfaithfulness. The fact that these words are not synonyms of each other and are not interchangeable is seen within the multiple passages that use them side-by-side: Matthew 15:19; Mark 7:21; Galatians 5:19. This fact can also be seen in the use of pornos and moichos side-by-side: 1 Corinthians 6:9; Hebrews 13:4. If porneia includes adultery, then why not simply use that word alone? To use moicheia alongside it is redundant if porneia refers to all sexual sins including adultery.

So let's get our terms accurate:

Adultery = Illicit sexual acts committed by married persons. (Post-marital)

Fornication = Illicit sexual acts committed by unmarried persons. (Pre-marital)

It is imperative to know that the Bible does not combine all sexually immoral behaviours, such as homosexuality, bestiality, pedophilia, prostitution, harlotry, etc., into the word "fornication." This is a false claim by many Bible teachers. Look up the 7 occurrences of porneuo, the 10 occurrences of pornos, the 12 occurrences of pornee, and the 25 occurrences of porneia in the New Testament and let the Bible define the meaning of these words. Likewise, look up the 4 occurrences of moicheia, the 4 occurrences of moichos, the 6 occurrences of moichaomai, and the 13 occurrences of moichuo in the New Testament and let the Bible define the meaning of these words. Therefore:

Adultery is only applicable to married persons.

Fornication is only applicable to unmarried or single persons.

The use of "sexual immorality" in modern translations of the Bible as opposed to "fornications" in ancient translations of the Bible is deliberately misleading. "Sexual immorality" obscures the meaning and intent of the passage while "fornications" makes it clear and precise. We need to remember that the Gospel According to Matthew was written to and for the Jews. Therefore, we need to understand their customs and practices in order to interpret it correctly. We cannot eisegetically impose our customs and practices upon the text and make it say what we want it to say.

The fact is, divorce is never okay. "For I hate divorce,' says the Lord, the God of Israel" (Mal. 2:16). This is further underscored by Paul when conveying God's words: "To the married I give instructions, not I, but the Lord, that the wife should not divorce her husband (but if she does divorce, she must remain unmarried, or else be reconciled to her husband), and that the husband should not divorce his wife" (1 Cor. 7:10-11). The words of Jesus solidify it: "From the beginning of creation, God MADE THEM MALE AND FEMALE. FOR THIS REASON A MAN SHALL LEAVE HIS FATHER AND MOTHER, AND THE TWO SHALL BECOME ONE FLESH; so they are no longer two, but one flesh. What therefore God has joined together, man cannot separate" (Mark 10:6-9). Marriage is a sacred covenant union instituted by God, entered into with binding vows, and intended for lifelong permanence. The only dissolution to the marriage tie is death (Rom. 7:2-3; 1 Cor. 7:39).

Any attempts at making other passages contradict the above is entirely disobedience and rebellion. For example, Paul makes it clear in 1 Corinthians 7:10-11 that these words come from God Himself. Therefore, Paul's personal words that follow in verses 12-16, 27-28 cannot and will not contradict God's own words. Any false teacher who tries to teach otherwise should be noted and avoided.