The principle of ‘one man, one wife’ was established with Adam and Eve in Genesis.
Genesis 2:18 And the LORD God said, It is not good that the man should be alone; I will make him an help meet for him.
Genesis 2:24 Therefore shall a man leave his father and his mother, and shall cleave unto his wife: and they shall be one flesh.
The principle was disregarded and under the Law of Moses was permitted and legislated for.
Kings were counselled against multiplying wives.
Deuteronomy 17:17 Neither shall he multiply wives to himself, that his heart turn not away:
Solomon is an example of one who ignored the counsel and suffered the consequences.
1 Kings 11:3 And he had seven hundred wives, princesses, and three hundred concubines: and his wives turned away his heart. 4 For it came to pass, when Solomon was old, that his wives turned away his heart after other gods: and his heart was not perfect with the LORD his God, as was the heart of David his father.
The principle of ‘one man, one wife’ was re-established in the New Testament.
Ephesians 5:30 For we are members of his body, of his flesh, and of his bones. 31 For this cause shall a man leave his father and mother, and shall be joined unto his wife, and they two shall be one flesh.. 32 This is a great mystery: but I speak concerning Christ and the church.
One man and one woman joined together in marriage represent the union of Christ and his bride. This principle had been hidden (a great mystery) until Christ gave his life for his bride.
2 Corinthians 11:2 For I am jealous over you with godly jealousy: for I have espoused you to one husband, that I may present you as a chaste virgin to Christ.
Ephesians 5:25 Husbands, love your wives, even as Christ also loved the church, and gave himself for it... 28 So ought men to love their wives as their own bodies. He that loveth his wife loveth himself. 29 For no man ever yet hated his own flesh; but nourisheth and cherisheth it, even as the Lord the church:
The principle is made a rule for the ordination of first century elders, bishops and deacons, to set an example to the flock.
1 Timothy 3:2 A bishop then must be blameless, the husband of one wife...
1 Timothy 3:12 Let the deacons be the husbands of one wife...
Titus 1:5 For this cause left I thee in Crete, that thou shouldest set in order the things that are wanting, and ordain elders in every city, as I had appointed thee: 6 If any be blameless, the husband of one wife...
Countries that espoused Christianity as a State religion made bigamy an offence under their secular laws. The marriage vows bind one man and one woman for life.
In abiding by the principle, a baptised believer exhibits the faithfulness of Christ and his bride. The same principle applies in the prohibition of divorce.
Having said this, if a man having more than one wife comes to believe the gospel and is baptised, there is no requirement in the New Testament for him to divorce his other wives. It is a principle to be observed not a law to be enforced by divorce.
I hope you have found this helpful.
May God bless you,