Is the polygyny of the Old Covenant allowed in the New Covenant age?
Polygyny is the practice of a husband having more than one wife at the same time. Polygyny was legal under the Sinai (Old) Covenant (note that some Christians dispute this). It may not have been an ideal family structure, but marrying a second wife was not considered to be adultery on the part of the husband. This is an important fact when understanding Jesus' teaching on the subject.
The important question, from a civil government perspective, is the following:
- Should the practice of polygyny be treated by civil government the same as the crime of adultery?
Is polygyny adultery?
In other words, Biblical law protects the marriage covenant by requiring the death penalty. Witnessed acts of adultery require punishment, by stoning, of both parties (Lev. 20:10, Deut. 22:22-24, John 8:7). Unwitnessed (but known) adultery could allow a husband to divorce his wife (Deut. 24:1, Matt. 1:19, Matt. 19:9). This shows the seriousness with which God treats the marriage covenant. The family is the foundation of civilization. The marriage covenant is the foundation of the intact family. Thus God protects the marriage covenant by the ultimate civil penalty (if the adultery is witnessed) and the possibility of divorce (as a deterrent to unfaithfulness).
If God meant for polygyny to be considered a type of adultery, then we would see a death penalty explicitly associated with it in Biblical law. No such penalty exists, and there are clear cases where polygyny is allowed under the Sinai Covenant: Exodus 21:10-11.
There was nothing immoral about polygyny under the Old Covenant. So neither Jacob  nor David were sinning by having more than one wife simultaneously. Of course the ideal form of marriage is clear from:
- Teleology: God designed childbirth such that equal numbers of men and women are born, on average. He could have designed it otherwise.
- The example of God's creation: Adam and Eve. One man, one woman, one marriage covenant.
Why did God allow polygyny, if it isn't the ideal?
Generally, you will find that polygyny is most often associated with cultures in which there is a lot of warfare. Frequent warfare has a tendency to create an imbalance of men and women. Polygyny, in such cultures, protects war widows from exploitation by allowing them to enter into the legal protection of a marriage covenant.
So the important next question is:
- Has the allowance of polygyny changed in the New Covenant?
I believe that Jesus, in Matthew 19:9, excludes polygyny from the New Covenant. Here's why:
Adultery is a sexual union which takes place outside of the legal protection of a marriage covenant, where one of the parties already has a valid marriage covenant. Polygyny establishes simultaneous marriage covenants, which protects the union of a man with two (or more) different women. As we saw above, polygyny| (under the older legal standard) could never be labeled as "adultery", because all the parties were legally protected under valid marriage covenants.
When asked about divorce in Matthew 19, Jesus responded with a statement that must have shocked his hearers:
9 I tell you that whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and he who marries her when she is divorced commits adultery.” Matthew 19:9WEB
Jesus says that only fornication could break the original marriage bond. Without an act of fornication (the exception), the original marriage covenant is still valid and unbroken. Therefore Jesus (on his authority as Lord of the Universe) is labeling polygynous re-marriage (marriage with an existing marital bond in place) as "adultery."
Whereas in the Old Covenant, polygyny was implicitly allowed (because it was not forbidden), Jesus now implicitly forbids it, by labeling simultaneous marriage covenants as "adultery." From a legal standpoint, this change would affect anyone who formed a marriage covenant after becoming aware of this addition to God's law.
Why did Jesus add this restriction to the law?
Here's my speculation. After bringing the Israelites out of slavery, God knew that they would be fighting lots of battles against the people in the land of Canaan. God directly ordered them to do this: Exod. 23:31-33. Anticipating warfare and the resulting imbalance of men and women, God allowed polygyny for a time, in order to protect women.
The New Covenant, however, was intended by Jesus to create a culture which is not spread through the force of arms. Christians are to avoid warfare as much as possible:
18 If it is possible, as much as it is up to you, be at peace with all men. Romans 12:18WEB
Therefore, it makes sense that polygyny could be safely excluded from the New Covenant. This is just my opinion, of course.
An important issue to divide over
I think this is an issue upon which Christians can legitimately differ. I would not withhold the hand of fellowship from someone who believed differently about Matt. 19:9. However, any given theonomic jurisdiction must enforce only one interpretation of this issue. Polygyny is either:
- the same as adultery, in which case witnessed "polygynous acts" are punished with the death penalty, or
- not adultery, and thus allowed by civil government.
Christians who differ on this issue must therefore set up two different jurisdictions/civil governments. There is no reason why they couldn't live at peace with one another despite this difference.
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- See also the question: What was the purpose of the Numbers 5 dusty(or "bitter") water test?
- Some people might object that Jacob preceded the law given under the Sinai Covenant. This is true, but irrelevant, because God revealed his laws prior to Sinai, and everyone recognizes that Jesus, in Matt. 19, appeals to this revelation of law which preceded Sinai.