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Pergunta: Por que Brigham Young iniciou a restrição ao Sacerdócio?
Question: Why did Brigham Young initiate the priesthood ban?
The start of Brigham Young's administration saw a continuation of Joseph Smith's policies
The start of Brigham Young's administration saw a continuation of Joseph Smith's policies. William McCary was baptized and ordained at Winter Quarters in October 1846. The following March, Brigham acknowledged the validity of the ordination of Walker Lewis that likely occurred during Joseph's tenure, "we [have] one of the best Elders an African in Lowell [,MA] -- a barber." 
The priesthood ban became more comprehensive to include not only slaves and free blacks in the South, but all persons deemed to have inherited the curse of Cain through Ham
The priesthood ban then became more comprehensive to include not only slaves and free blacks in the South, but all persons deemed to have inherited the curse of Cain through Ham. Three pivotal events in this development were the apostasy of William McCary, the interracial marriage of Walker Lewis's son, and the passing of slavery legislation in Utah Territory.
McCary approached Brigham Young with complaints that racial discrimination was a motive behind other Mormon leaders questioning his strange teachings. President Young satisfied McCary that ideally race should not be the issue. Praising Walker Lewis as an example, Young suggested "Its nothing to do with the blood for [from] one blood has God made all flesh" and later added "we don't care about the color."  Shortly thereafter McCary was excommunicated for apostasy. In April, Brigham Young departed with the vanguard pioneer company for the Rocky Mountains only to return around December to face additional race-based problems.
In April, Elder Parley P. Pratt had warned of the Saints about following schisms led by those like James Strang and William McCary. Significantly he referred to William McCary as "this black man who has got the blood of Ham in him which linege was cursed as regards the priesthood". McCary had married a Stake President's white daughter and advocated polygamy before his excommunication and afterward he began drawing away Mormon women to be sealed to him in a carnal manner.
Brigham was adamantly against racial amalgamation
Also awaiting Brigham was William Appleby, the president over eastern branches of the Church. He had encountered the Lewises and suspected William Smith had acted improperly by ordaining a black elder. He was also alarmed that Enoch Lewis had married a white wife and had a child. Brigham responded to this news in a manner that is, by modern sensitivities, quite disturbing. He was adamantly against racial amalgamation (see Brigham Young on race mixing for more context). While allowing that interracial couples should not be denied baptism, he introduced a ban on temple service for them and/or their offspring.
Brigham Young never presented a specific revelation on priesthood or temple restrictions he imposed
However, Brigham Young did not present a specific revelation on priesthood or temple restrictions he imposed. A definitive statement wasn't made by him until 1852 in a legislative, rather than ecclesiastical forum. Governor Young declared "any man having one drop of the seed of [Cain] ... in him cannot hold the priesthood and if no other Prophet ever spake it before I will say it now in the name of Jesus Christ I know it is true and others know it."  Like the Missouri period, the Saints were externally pressured to adopt racial policies as a political compromise. At the time, this was deemed to be the best pathway to statehood.
Those who believe the ban had a revelatory basis point to these pivotal events as examples of a prophet learning "line upon line," with revelation being implemented more rigorously. Those who see the influence of cultural factors and institutional practice behind the ban consider this evidence that the ban was based on Brigham's cultural and scriptural assumptions, and point out that such beliefs were common among most Christians in Antebellum America.
- Church Historian's Office. General Church Minutes, 1839–1877, March 26, 1847, in Selected Collections from the Archives of the Church of Jesus Christ of Latter-day Saints, 2 vols., DVD (Provo, Utah: BYU Press, 2002), 1:18.
- General Church Minutes, March 26, 1847.
- General Church Minutes, April 25, 1847.
- Neither White nor Black, 70–72.
- For a history of such ideas in American Christian thought generally, see H. Shelton Smith, In His Image, But...: Racism in Southern Religion, 1780–1910 (Durham, North Carolina: Duke University Press, 1972), 131. ISBN 082230273X.