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From MormonWikiThe Church of Jesus Christ of Latter-day Saints ( Mormons) is the President of the Church, whom the members revere as the prophet, seer, and revelator in a biblical sense. He is entitled to receive revelation from God to guide the Church and the world as His mouthpiece on the earth. The President of The Church of Jesus Christ of Latter-day Saints is the only living person who holds all of authority (or keys) necessary for individuals to receive the ordinances of salvation and become worthy of entering into the presence of the Lord, and to receive the gift of eternal life. Eternal life refers to the quality of life lived in the highest degree of glory--or celestial kingdom--after this life.
The President of the Church serves as such until death, after which the Quorum of the Twelve Apostles meet, pray, and under the leadership of the senior apostle, receive revelation as to whom the Lord chooses as His next prophet. Although not specified by revelation, the senior apostle has historically become the new President of the Church. (This seniority is based on the date when the individual was called as an apostle, not his calendar age.) Bold text