Commonplace in Christian theologies is the belief in a Second Coming. According to this notion, Christ will return from heaven to earth in accord with the Messianic prophecy that claims he will resurrect the dead, render a last judgment of the dead, and establish a Kingdom of Heaven on Earth. Thus, “Adventists” subscribe to this notion of the “Second Advent of Jesus.”

The term “Adventist” is used interchangeably with the “Seventh-day Adventist Church.” The Seventh-day designation serves as a reminder that their Sabbath is distinguished by its observance on Saturday, the seventh or last day of the week, instead of a Sunday Sabbath. In fact, the Adventists, like Jews, observe the Sabbath from Friday sunset to Saturday sunset. According to the creation story in the Bible, Saturday was instituted by God as the day of rest, and the commandment concerning Sabbath rest is part of God's eternal law.

Adventism is one of a group of Protestant Christian churches with a belief in the Second Coming, with firm roots in the Hebrew and Christian prophetic tradition. They believe that when Christ returns he will separate the saints from the wicked. The founder of the faith, William Miller (1782–1849), was a Baptist preacher. His followers were called Millerites. Miller came to the conclusion that Christ would arrive sometime between March 21, 1843, and March 21, 1844. Though he was encouraged in this view by some clergymen and followers, Miller was also accused of being a fanatic because he insisted that Christ would arrive on schedule with a fiery conflagration.

Christ did not appear as predicted, so Miller set a second date: October 22, 1844. So great was the expectation of Christ's arrival that Millerites changed their ways of life, and, in many cases, abandoned their possessions and joined Miller in awaiting the arrival. That day passed quietly, too, and followers suffered what was called the Great Disappointment by Adventists. Nevertheless, Millerites persisted; they believed Miller had set the right date, but it had been interpreted incorrectly. Some members of the faith independently believed that Christ's advent was still imminent, although they didn't set a new date.

Later, Adventists reinterpreted the events of 1843–1844 and claimed that something had occurred in the invisible realm, where Jesus lived with his Father. So Adventists again expected Jesus to come. Times of great religious experiment ensued, and Adventists promulgated vegetarian and other measures to improve health.

In the mid-1990s, membership numbered 733,000 in the United States, but the worldwide figure has swelled to nearly 5 million.