Polygamy was allowed in the Old Testament because they were under the law. The ten commandments were given to Moses to give to the people. After Christ was born in the New Testament, died and was resurrected then that did away with the 'law.'
The Bible forbids multiple wives for a few reasons— committing adultery and the opportunity for one's heart to be turned to other gods.
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. 1 Kings 11:4
Neither shall he multiply wives to himself, that his heart turn not away: neither shall he greatly multiply to himself silver and gold. Deuteronomy 17:17
Now, we are under 'grace,' and not under the 'law.'
Know ye not, brethren, (for I speak to them that know the law,) how that the law hath dominion over a man as long as he liveth? For the woman which hath an husband is bound by the law to her husband so long as he liveth; but if the husband be dead, she is loosed from the law of her husband.
So then if, while her husband liveth, she be married to another man, she shall be called an adulteress: but if her husband be dead, she is free from that law; so that she is no adulteress, though she be married to another man.
Wherefore, my brethren, ye also are become dead to the law by the body of Christ; that ye should be married to another, even to him who is raised from the dead, that we should bring forth fruit unto God.
For when we were in the flesh, the motions of sins, which were by the law, did work in our members to bring forth fruit unto death. But now we are delivered from the law, that being dead wherein we were held; that we should serve in newness of spirit, and not in the oldness of the letter. Romans 7:1-6
In fact, keeping the Sabbath is not in the New Testament because we are supposed to treat every day the same.
Let no man therefore judge you in meat, or in drink, or in respect of an holyday, or of the new moon, or of the sabbath days: Which are a shadow of things to come; but the body is of Christ. Colossians 2:16-17
Read Romans 14, the law of liberty and about esteeming every day alike. The law becomes a burden and a curse until we allow it to point us to Christ, who fulfills all its requirements.
For the law of the Spirit of life in Christ Jesus hath made me free from the law of sin and death. For what the law could not do, in that it was weak through the flesh, God sending his own Son in the likeness of sinful flesh, and for sin, condemned sin in the flesh: That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit. Romans 8:2-4
For Christ is the end of the law for righteousness to every one that believeth. Romans 10:4
Let love be without dissimulation. Abhor that which is evil; cleave to that which is good. Be kindly affectioned one to another with brotherly love; in honour preferring one another; Not slothful in business; fervent in spirit; serving the Lord; Rejoicing in hope; patient in tribulation; continuing instant in prayer; Distributing to the necessity of saints; given to hospitality.
Bless them which persecute you: bless, and curse not. Rejoice with them that do rejoice, and weep with them that weep. [Be] of the same mind one toward another. Mind not high things, but condescend to men of low estate. Be not wise in your own conceits. Recompense to no man evil for evil. Provide things honest in the sight of all men. If it be possible, as much as lieth in you, live peaceably with all men.
Dearly beloved, avenge not yourselves, but [rather] give place unto wrath: for it is written, Vengeance [is] mine; I will repay, saith the Lord. Therefore if thine enemy hunger, feed him; if he thirst, give him drink: for in so doing thou shalt heap coals of fire on his head. Be not overcome of evil, but overcome evil with good.
NR Caetta is a 1961 graduate from Zion Bible Institute in East Providence, Rhode Island where she studied Bible Theology.