Lynne W. Jorgensen summarizes a unique event in church history:
On August 8, 1844, six weeks after the Prophet Joseph Smith’s martyrdom, a meeting of the Saints was held in Nauvoo, Illinois. Brigham Young, President of the Quorum of the Twelve, and several other Apostles had just returned from missions. The purpose of the meeting was to determine by vote who had the right and responsibility to lead the Church—Sidney Rigdon, First Counselor in the First Presidency, or the Quorum of the Twelve with Brigham Young at their head. In the course of the two meetings held that day, many in attendance received a divine witness that Brigham Young was to be the next leader: some Saints specifically state that as Brigham Young addressed the congregation he sounded and appeared remarkably like Joseph Smith, others simply say that the “mantle of Joseph” or “of the prophets” rested on Brigham Young, and others state that they were given a witness “by the spirit” that Brigham was to lead the Church.
This spiritual experience, which has come to be known as “the mantle of the Prophet falling on Brigham Young,” served to unite and comfort the Saints as they mourned the death of their beloved prophet and to direct them when they voted to support Brigham and the Twelve as leaders of the Church. Some Saints who were not present at the August 8 meetings reported experiencing a similar “mantle” witness on later occasions. At least one hundred people wrote or otherwise passed on their remembrances of this witness.
– Lynne W. Jorgensen – The Mantle of the Prophet Joseph Passes to Brother Brigham – A Collective Spiritual Witness
One first-hand testimony is that of William Adams:
There was a great multitude attending the meeting, more than one half the crowd could not find seats, and stood on their feet. Never were so many at one meeting that I ever saw. I was sitting down and could not see the speakers on the stand. I was listening very attentively, so that I could hear every word.
I heard a voice speaking, I was surprised, and jumping to my feet, expecting Joseph the Prophet was speaking, having heard him often in public and private, so that I was quite acquainted with his voice. This was a strong testimony that the Twelve Apostles were the rightful leaders of the church and that the mouth of Joseph had fallen on Brigham Young.
– Autobiography of William Adams
Why are there so many accounts of this taking place?
See: