What Christians were polygamists? Maybe the Mormons if they can be considered Christian. Anyone else?
Middle Ages:
In the medieval period, multiple wives were often obtained through kidnapping. It is with this in view that we must interpret the following laws: The Frankish Laws of 818-9 strictly forbade kidnapping of women.[33] The XXVII. law issued by King Stephen I of Hungary (10001030) declares that the kidnapper must return the woman to her parents even if he has had sexual intercourse with her, and must pay a penalty to the parents. According to the Hungarian law, the kidnapped girl was then free to marry whomever.[33]
The Roman councils of 1052 and 1063 suspended from communion those laymen who had a wife and a concubine at the same time.[34] Divorce was also forbidden, and remarriage after a divorce counted as polygamy. Nicholas the Great (858-67) forbade Lothair II of Lotharingia to divorce his barren wife Teutberga and marry his concubine Waldrada, with whom he had several children. After a council of the Lotharingian bishops, as well as the archbishop of Köln and Trier had annulled his marriage to Theutberga, the pope voided this decision, and made him take his wife back.[35][36]
In Scandinavia, the word for an official concubine was "frille". Norwegian Bishop Øystein Erlendsson (ca. 1120-1188) declared that concubines were not allowed to accept the sacraments unless they married, and men were forced to promise marriage to women they had lain with outside of wedlock. In 1280, the Norwegian king Eirik Magnusson (128099) declared that men were exempted from having to promise marriage to the frille, if they went to confession and did penance. The Church answered by making several declarations in the 14th century, urging men to marry their concubines. In 1305, King Håkon V (12991319) issued a law that declared marriage to be the only lawful way of cohabitation, and declared that only women in wedlock were allowed to dress as they pleased, while the dress of concubines was restricted.
Reformation period:
While monogamy was the norm among Christians,[38][39] in the 16th century there was a Christian re-examination of plural marriages. The founder of the Protestant Reformation, Martin Luther wrote: "I confess that I cannot forbid a person to marry several wives, for it does not contradict the Scripture. If a man wishes to marry more than one wife he should be asked whether he is satisfied in his conscience that he may do so in accordance with the word of God. In such a case the civil authority has nothing to do in the matter."[40]