The Millerites were a 19th-century North American religious movement who believed that Jesus Christ would return to Earth between 1843 and 1844.

Core Beliefs

• Led by the Baptist preacher William Miller.
• Based on studies of the biblical books of Daniel and Revelation.
• Interpreted the 2,300-day prophecy in Daniel 8:14 to mean the Earth would be cleansed by fire upon Christ’s return.
• Grew popular during the religious revival known as the Second Great Awakening.

The Great Disappointment

• Followers anticipated the Second Coming on October 22, 1844.
• When Jesus did not appear, the event became known as the Great Disappointment.
• The failed prediction caused many followers to leave the movement.

Legacy

• Remaining adherents formed the foundation of modern Adventism. Led directly to the creation of denominations like the Seventh-day Adventists and Advent Christians. This went on to invent Young Earth Creationism.
• You can read more historical background on the Adventist Encyclopedia.

From the Adventist Encyclopedia:
Miller studied the prophecies of Daniel and Revelation, the year-day method equating a day in Bible prophecy to a literal year. He deduced from the 2,300 days of Daniel 8:14 that this period began with Artaxerxes’s decree in 457 B.C. to go forth and rebuild Jerusalem. He interpreted the “cleansing of the sanctuary” as referring to the purification of the earth by fire when Jesus Christ returned the second time. By 1818, after more than two years of intense study, he concluded that Christ would return in about 25 years.

Also:
Scholars of Adventism have struggled to identify the socio-economic demographics of Millerites.  David L. Rowe observed that “Millerites are not fascinating because they were so different from everyone else but because they were so like their neighbors.”  Although typical of American Christianity, the movement was also “eclectic and diverse” showcasing how leaders came from “all classes . . . all sects, nations, and tongues.”  Lists of Millerite believers in periodicals suggest that they came from a wide range of socio-economic backgrounds, perhaps even slightly above average in terms of wealth usually measured in land holding. Millerites therefore represented a cross-section of North American society. A breakdown of 174 Millerite preachers by Everett Dick provides some indication of religious affiliation amongst the different denominations: Methodism (44%), Baptist (27%), Congregational (9%), Christian Connection (18%), and Presbyterian (7%).  Kevin M. Burton, studying approximately 1,800 Millerite believers, identified 35.3% as Baptist; 32.6% as Methodist; 15.3% as Restorationist; and 12.7% as Presbyterian or Congregationalist; and 4% for various others including Dutch Reformed, Quakers, etc.²⁵ Millerites worked diligently to distinguish themselves from other religious groups, most notably the Church of Jesus Christ (Mormons) even though there were a few Mormons who converted to Adventism. In order to avoid any confusion, Joshua V. Himes issued a tract to “fully expose the iniquity of this [Mormon] delusion.”