Pat asked: “Is polygamy a sin?"
I need to begin with the obvious acknowledgment that the Bible describes men in polygamous relationships. You have men such as Abraham, David, and Solomon—all with multiple wives. Some might read that and come to the conclusion that polygamy is acceptable.
Keep in mind that not everything the Bible reports is recommend or endorsed. The Bible never openly recommends polygamy. It just reports that it happened, and that Old Testament believers participated in it.
We know that polygamy is a digression from God's original design. He created one man and one woman, introduced them to one another, and joined them in a marriage. That’s how God’s Word defines marriage. We leave our father and mother, cleave to our spouse (singular), and become one flesh.
Abraham’s polygamy reflected a lack of faith in God and brought bitterness, jealousy and animosity to his household. Jacob’s polygamy brought enmity, anger and jealousy between Rachel and Leah. Solomon's polygamy brought him spiritual destruction. So again, the Bible tells us that they did this, but it does not commend their behavior.
The kings of Israel were told directly not to multiply wives in Deuteronomy 17. It was forbidden. David, Solomon and others violated that command.
When you come to the New Testament, Jesus validated the definition of marriage as between one man and one woman in Matthew 19. When you get into the epistles, you find that it is absolutely required for the pastor, elder, or deacon to be the husband of one wife.
Finally, I’ll point out that marriage is designed as a wonderful illustration of Christ and the church. The church is called the bride of Christ. We represent one bride and we have one husband. That mystery between Christ and the church, Paul tells us in Ephesians, is illustrated in the uniqueness of a monogamous relationship between a husband and a wife.
Thanks for your question,