Despite the many examples of polygamy in the Bible, it was never God's intent for people. He created Adam and Eve to cleave together, the two becoming one (Genesis 2:24). He also gave them the joint mission to rule over and fill the earth (Genesis 1:28). Once sin entered the world, that mission and the way in which it could be fulfilled, was warped. People began to value God's gifts less. One of those gifts was a committed marriage relationship between a man and woman who were devoted to each other and God.
Why did God allow polygamy in the Bible?
Other concerns started to take priority. The once fertile ground was covered with weeds, leading families to value children who could work in the fields. Cain's murder of Abel set the precedence for selfishness and violence instead of dedication to God's calling. Women lost standing as joint-heirs of God's blessing and instead served to witness man's ambition (Genesis 4:23-24) and provide sons to advance it.
Several centuries later, society had degraded even further. Many cultures taught that men were to rule God's creation. Women became almost superfluous except as a means to make baby men. A woman on her own was thought to have no contribution to society. She had no protection from violence. No role in the community. And often the only way she could provide for herself was through prostitution.
Men, on the other hand, worked toward God's mission to rule by acquiring and holding onto land and property. In the patriarchal society, this was done through sons who could work the land and daughters whose marriages could cement political alliances. In many cultures, there was no socio-political value to unmarried, unrelated women.
This was not God's intent. God created women to, married or not, join with men in the mission to subdue and fill the earth. Only after Eve was created did God look over creation and say it was "very good" (Genesis 1:31). But God also gave mankind the free will to be able to warp His plan. He could not have reached in and corrected culture's view of women without retracting that free will. So He worked within the culture instead.
As strange and odd as it sounds, polygamy in the Bible protected women. A second or third wife would be far more cared for than an unmarried woman. She would have the opportunity to bear children—especially sons who would be responsible for supporting her in her old age. She would have protection from abuse from strangers. And she would have a home. She might also have the chance to rescue a family. If the first wife was barren, her sons could continue the family name (1 Samuel 1:2).
On rare occasions, the Bible shows how God even used polygamy as a reward of protection for a faithful woman. David married Abigail after she showed great wisdom in the face of her foolish husband, who later died (1 Samuel 25). It's been speculated that Ruth was significantly younger than Boaz because he called her "daughter" (Ruth 2:2). If so, such a powerful land-owner surely would have had other wives.
Although the Bible hints that protection of women is probably the main reason why God allowed polygamy, most polygamous marriages were not so altruistic. Marriages were very often contracts between families. Solomon surely loved women (1 Kings 11:3), but it's safe to say most of his wives were given to him to gain political favor. And they were not beneficial for Solomon's walk with God or Israel.
The Bible tells us that polygamy was not God's original plan. It is not the ideal of marriage. The fact that it was deemed necessary to protect women and maintain the holdings of men is a commentary on humanity's ability to twist God's plan. God did not intend for women to be useless and worthless without a man. When society decided they were, God acted to protect His daughters.
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Truth about Marriage