For new readers (and old) I prepared this brief background on Letter VII that you can use to explain to other people.
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Letter VII background
With the assistance of Joseph Smith, Oliver Cowdery wrote a series of eight letters about the early history of the Church. They were initially published in the Messenger and Advocate in 1834-1835. Part of Letter I is included in the Pearl of Great Price. Letter VII is especially noteworthy because it declares it is a fact that the Hill Cumorah in New York was the scene of the final battles of the Jaredites and the Nephites. Letter VII also specifies that Mormon’s depository was located in the same hill, a teaching later reaffirmed by Brigham Young, Wilford Woodruff, Heber C. Kimball, Orson Pratt, and others.
Shortly after the letters were published, Joseph directed his scribes to copy all eight letters into his personal history (History, 1834-1836, found at http://www.josephsmithpapers.org/paper-summary/history-1834-1836/83).
In 1840, Orson Pratt reprinted portions of the letters (including Letter VII) in his pamphlet, “A[n] Interesting Account of Several Remarkable Visions.” Joseph later adapted portions of this pamphlet when he wrote the Wentworth Letter in March 1842, although he replaced Pratt’s hemispheric concept with the simple statement that “The remnant are the Indians that now inhabit this country.”
Joseph desired that all members of the Church be aware of these letters. In the fall of 1840, Joseph gave them to his brother, Don Carlos, to reprint in the Times and Seasons. Letter VII was published in the April, 1841, edition of the Times and Seasons. Also in 1840, Joseph gave permission to Benjamin Winchester to reprint the letters in the Gospel Reflector, Winchester’s Mormon newspaper in Philadelphia. All eight letters were printed as a special edition of the Gospel Reflector in March 1841.
Responding to strong demand, the eight letters were reprinted as a pamphlet in England in February 1844.
Beginning in May 1844, The Prophet newspaper reprinted the letters in New York City. William Smith reprinted Letter VII on June 29, 1844—two days after the martyrdom.
All eight letters were reprinted in the Millennial Star and the Improvement Era. All of Joseph’s contemporaries and successors accepted Letter VII’s teachings about the Hill Cumorah in New York.
However, beginning in the 1920s, RLDS scholars reassessed the Book of Mormon and decided the narrative took place in a limited area of Central America. This meant that Cumorah, too, was actually somewhere in Southern Mexico. LDS scholars gradually adopted the same rationale.
Alarmed at the development, Joseph Fielding Smith, then Church Historian and an Apostle for 20 years, declared:
“This modernistic theory of necessity, in order to be consistent, must place the waters of Ripliancum and the Hill Cumorah some place within the restricted territory of Central America, notwithstanding the teachings of the Church to the contrary for upwards of 100 years. Because of this theory some members of the Church have become confused and greatly disturbed in their faith in the Book of Mormon.”
When he was President of the Quorum of the Twelve, Joseph Fielding Smith reissued his warning about the two-Cumorahs theory. However, LDS scholars and educators rejected his counsel, claiming it was merely his opinion and their own ideas were correct. Even now, in 2017, LDS scholars and educators actively teach that Joseph and Oliver were ignorant speculators who misled the Church about Cumorah being in New York. They teach that Joseph adopted a false tradition about Cumorah, and that all the Prophets and Apostles who reaffirmed the teaching of Letter VII were also expressing personal opinions—even when they spoke in General Conference.
The influence of these scholars permeates the Church. The two-Cumorahs theory is now being taught at BYU (where it is an integral component of the required Book of Mormon classes), in CES, and in Visitors Centers throughout the Church. Unlike in Joseph’s day, few Church members even know about Letter VII.
As President Smith warned, the two-Cumorahs theory has led many thousands of members of the Church—especially the youth—to lose their faith. It is an obstacle many investigators cannot overcome. The tragedy is Joseph and Oliver answered this question all the way back in 1835 and yet LDS scholars reject them.
Source: Letter VII