Osborn J. P. Widtsoe writes that Joseph was receiving "revelations" of plural/eternal marriage as early as 1831.

Osborne J. P. Widtsoe

Osborne J. P. Widtsoe, The Restoration of the Gospel (Salt Lake City, UT: Deseret News, 1912), 157–158

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Joseph Smith, Jr., Osborne J. P. Widtsoe
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There is abundant evidence that the Prophet Joseph Smith received probably as early as 1831 revelations on these important questions. Because, however, the people were not yet prepared to receive the great truths that had been revealed to him, the revelations were withheld for many years. In a revelation of the year 1831, the Prophet said only, concerning marriage, "that whoso forbiddeth to marry is not ordained of God, for marriage is ordained of God unto man; wherefore it is lawful that he should have one wife, and they twain shall be one flesh, and all this that the earth might answer the end of its creation."

It was not until July 12, 1843, that the glorious truths concerning the sanctity and eternity of the marriage covenant—long before revealed to the Prophet Joseph Smith—were committed to writing and read by his friends. Even then, not all that had been given to him was revealed to his followers. But in the publication of as much as he gave, another step in the great drama of the Restoration was enacted. Owing to the persecutions to which the early Saints were subjected, the revelation was not officially published until 1852, five years after the wearisome tramp across the plains to the Rocky Mountains.

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