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Pergunta: Porque não mórmons atualmente praticar o casamento plural, isso significa que os primeiros líderes que ensinaram que é foi exigido estavam errados?
Question: Because Mormons do not currently practice plural marriage, does this mean that early leaders who taught that is was required were wrong?
The purpose of modern prophets is to give the Saints the will of God in their particular circumstances
Critics of the Church ignore that the purpose of modern prophets is to give the Saints the will of God in their particular circumstances. Joseph Smith wrote specifically of the issue of plural marriage:
This is the principle on which the government of heaven is conducted—by revelation adapted to the circumstances in which the children of the kingdom are placed...in obedience there is joy and peace unspotted, unalloyed; and as God has designed our happiness—and the happiness of all His creatures, he never has—He never will institute an ordinance or give a commandment to His people that is not calculated in its nature to promote that happiness which He has designed, and which will not end in the greatest amount of good and glory to those who become the recipients of his law and ordinances. 
LDS doctrine also holds that the prophet, when speaking in an official capacity, speaks on behalf of the Lord:
whether by mine own voice or by the voice of my servants, it is the same. (D&C 1:38)
Critics of the Church often come out of an inerrantist background, or draw on arguments first formulated by religious inerrantists or fundamentalists. In an inerrantist religion, God's instructions cannot change with circumstances—if they did, then the Biblical record would not be sufficient, on its own, to guide us. Since inerrantists require, above all, that the Bible be the sole authority, they must assume that God's requirements are always the same.
However, even the Bible gives many examples of God giving new instructions because of new circumstances, or contravening previous instructions:
- Noah (but no other prophet) was to build an Ark (Gen. 6:14)
- Moses implemented the Passover, which was hitherto unknown (Ex. 3:12-28)
- Jesus revoked the celebration of Passover, and modified the ordinance and its performance at the Last Supper (Matt. 26:26, Mark 14:22, Luke 22:19)
- Hosea was commanded to marry a prostitute as a sign to Israel Hos. 1:1-3
- Jesus told his disciples only to preach to Israelites (Matt. 10:5-6, Matt. 15:24)
- The Lord later told the prophet (Peter) to preach to all people (Acts 10:14-28)
In each case, failure to obey carried significant penalties. Yet, when proper authority altered or rescinded a command, spiritual disaster followed those who did not obey the new instructions.
President John Taylor said:
Where did this commandment come from in relation to polygamy? It also came from God. It was a revelation given unto Joseph Smith from God, and was made binding upon His servants. When this system was first introduced among this people, it was one of the greatest crosses that ever was taken up by any set of men since the world stood. Joseph Smith told others; he told me, and I can bear witness of it, "that if this principle was not introduced, this Church and kingdom could not proceed." When this commandment was given, it was so far religious, and so far binding upon the Elders of this Church that it was told them if they were not prepared to enter into it, and to stem the torrent of opposition that would come in consequence of it, the keys of the kingdom would be taken from them. When I see any of our people, men or women, opposing a principle of this kind, I have years ago set them down as on the high road to apostacy, and I do to-day; I consider them apostates, and not interested in this Church and kingdom.