Brigham Young was Joseph's true successor because he held all Priesthood keys under Joseph. He was the presiding authority in the Council of 50. He was sealed as a son to Joseph under the Law of Adoption. Like Peter of old, he was the presiding authority of the church as president of the Quorum of Twelve. He was one of the first initiated into the "anointed quorum" and was privy to all the "secret doctrines" of the Nauvoo period, such as plural marriage, blood atonement, Joseph's being the King of all Israel, etc. Besides all of these things, he lived the rest of his life striving to establish Zion as Joseph Smith had. Indeed, the Lord has told us by the power of the Holy Ghost that Brigham Young and John Taylor kept the work of Joseph alive with Joseph's approbation.