JH> not to worry, I knew you weren't trying to discredit me; it was just my smarmy way of trying to get people to ponder something they hadn't thought of before.
Now, on to the nitty-gritty.
"Fornication", as used in JH's quote from Ephesians 5:5 (Be sure of this, that no fornicator or impure person, or one who is greedy (that is, an idolater), has any inheritance in the kingdom of Christ and of God), is our English translation of the Greek word porneia (a word which was used VERY rarely in secular Greek).
It's gonna get confusing here, folks -- hold on.
The Old Testament was originally an oral tradition, that was eventually written down in Hebrew. It is the Hebrew root "znh" which meant "fornication" (sometimes involving adultery -- it may be used of the prostitute or of the betrothed or married woman who proves unfaithful); figuratively it is a term for apostasy as unfaithfulness to God.
Already we can see that the word is carrying two meanings, depending on the context - a literal (prostitution) and a figurative (idolatry).
Now, in the inter-testamental period (that is, the time between the Old and New Testaments), many other works were written - some books are included in some versions of the Bible, but not in others (some call these books, like Sirach, "Apocryphal" books, whereas other groups like Catholics include them in the canon). In this interim timeframe, the Greek-speeking Jews of the Hellenistic world translated their Hebrew scriptures into Greek. In these versions, they took the Greek root "porne" to stand for the Hebrew root "znh", and expanded upon "porneia" to include not only fornication or adultery but incest, sodomy, unlawful marriage, and sexual intercourse in general.
Now we reach the time of the New Testament, approximately 20 to 70 years after the death of Jesus. Here's what we can figure from what we know of the years these books of the bible were written. I'll list items of interest in chronological order.
Paul's letter to the Corinthians (55AD): Idolatry and licentiousness are linked together in 1 Cor. 6:9, and serious though fornication is, there is forgiveness for it (1 Cor. 6:11).
Acts of the Apostles (60AD): There are three examples of porneia in 15:20, 29; 21:15. The apostolic council requires Gentile believers only to avoid four things, of which porneia is one.
Matthew (63AD): Jesus proclaims grace and forgiveness to all who repent; the true defilement is within (Mt. 15:18-19), and it is a mark of unbelief.
Paul's letter to the Ephesians (80AD): Interestingly enough, this book was probably written by someone who was *not* Paul, but tried to imitate his style & teachings. AS stated above, the quote is "Be sure of this, that no fornicator or impure person, or one who is greedy (that is, an idolater), has any inheritance in the kingdom of Christ and of God."
John (85AD): In 8:41 the Jews claim that they are not born of fornication (porneia), but their works show that they are children of the devil rather than of Abraham or God (v. 44).
Revelation (90AD): Written as a condemnation of first-century Rome, and not as a look at evil world powers in the 21st century, the writer of Revelation saw ancient Rome as the epicenter of "porne", that is, the center of paganism and a harlot-like apostasy from God.
What can we determine from this progression? That from the Old Testament onwards to the end of the New, the word for fornication is very often linked with the word for idolatry.
Is idolatry sinful behavior? Sure seems to be. And how did the ancients describe idolatry in a way that was clear-cut and concrete? By using the idea of fornication. Idolatry, which is a turning away from God (and thus sinful) is like fornication, especially when you view God/Jesus as a husband, and Israel/Judaism/Christianity as a wife. But is fornication in and of itself sinful? According to the bible, no, not always.
So then, what constitutes "sinful sexual behavior"? Look at it in two ways: what the Bible says, and what mainline churches say.
The Bible really doesn't concern itself about sexual behavior as such: it is instead concerned with right relationships. A sexual behavior is "wrong" or "immoral" if it interferes with the established order of things. One is not to "mix improperly" -- humans shouldn't perform sex acts with animals because that would blur the distinction between the two. Similarly with incest, one cannot blur family lines: a cousin is a cousin, and one should have a different relationship with that person than one would with one's wife. So it goes with marriages also.
So what is Biblical sinful sexual behavior? It is the same as Biblical sinful behavior in general: any behavior that upsets the order of things, the relationship between things, between people, and between people and God.
What is sinful sexual behavior according to many churches today? It depends on the church: certain sexual acts within a marriage; infidelity within a marriage; sex outside marriage; and homosexual sex are just an example of what some churches consider "sinful".
The question then becomes, is it sinful because sexuality is of the body, and the body is "bad" compared to things spiritual? Or do they say it's sinful because they understand fully the Biblical meaning behing sexual admonitions? My professional opinion is because of the former: a pseudo-Gnostic framework has been a part of Christianity since almost the very beginning, which says body bad, spirit good. And because sex is "of the body", it's bad, unless it's somehow "regulated".
I'm gonna stop here for a bit, in the hopes that I'm making sense to everyone.
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My knob tastes funny.