Restorationism

Restorationism is sometimes used by religion scholars to denote a fourth branch of Christianity (after Roman Catholicism, Eastern Orthodoxy and Protestantism), encompassing denominations which resemble Protestantism but do not have a direct lineage from the Protestant Reformation, nor, by extension, from the Catholic Church. Restorationist churches are those which began as entirely new churches, declaring all the existing churches apostate and that they were restoring the original church. Most of them were founded in the 19th and 20th centuries.

All other Christians are apostates
Typical of Restorationist beliefs is that the early church fell into complete apostasy, which they usually date to around 312 CE when the emperor Constantine of the Roman Empire either converted to Christianity or cynically co-opted it, depending on whom you ask. Some Restorationists will date this apostasy slightly earlier, to Tertullian's formulation of the doctrine of the trinity, or to the switch from Saturday to Sunday worship. The apostasy is often viewed as a deliberate introduction of paganism into the church, including (depending on whom you ask) the Christianising and renaming of pagan festivals to become Christmas and Easter, the introduction of goddess worship into the church under the guise of Mary, the acceptance of warfare and swearing oaths (both forbidden by the early church with oaths viewed as pagan rites), and/or the introduction of infant baptism and baptism by sprinkling rather than dunking. Examples of books purporting to trace exactly when and how the early church fell into apostasy are The Great Apostasy by Mormon leader James Talmage, and The Great Controversy by Seventh-day Adventist founder Ellen G. White. Other books, such as The Trail of Blood (a Baptist booklet) and the Anabaptist/Amish tome Martyr's Mirror, also hold to this belief that the church as a whole fell into apostasy, but that a smallish persecuted remnant of true believers remained underground and carried on the true faith in a line of succession until the present.

Restorationist groups
The Church of Christ, originally known as the Campbellites or the Restoration Movement, appears to account for the origin of the term "Restorationist". Some religious scholars use the term only to refer to the Campbellite movement and to Mormonism - due to the early connections between the two - and not to anyone else. Others employ a broader use of the term, including some or all of the following: Jehovah's Witnesses, Pentecostalism (especially its Jesus-only baptism branch), Christian Identity, Seventh-day Adventism and its offshoots (including the Branch Davidians and the  Worldwide Church of God),  the Plymouth Brethren, Joel's Army, the Discipling and Shepherding movement, the Twelve Tribes communities, and the Local Church movement. Older movements with origins on the radically low-church end of the Protestant Reformation, such as the Puritans, the Society of Friends (aka the Quakers) and the Anabaptists, are usually not included, even though they resemble in many ways the more recent Restorationist groups' attempts at restoring

A similar religious movement in the Philippines called Iglesia ni Cristo shares much of the core tenets with the above-listed groups, though it also incorporated concepts such as personality cults towards its leaders and bloc voting under the guise of fostering "solidarity" among the congregation. The same can be said about the Kingdom of Jesus Christ religious movement by Filipino far-right fugitive pastor Apollo Quiboloy, who employed not only a personality cult around himself but also engaged in human trafficking of women and girls where they are illegally brought to the States under false pretences.

Disagreement
It is notable that due to the nature of Restorationism, most of the groups so labelled have little in common with each other, other than declaring themselves to be the restoration of God's True Church™. They tend to differ greatly on which original practices they restore. One group may emphasize Saturday worship, another may emphasize Christian pacifism, another may emphasize home church meetings, no liturgical music and rejection of denominational titles.

Most of these groups are notoriously fundamentalist and cranky, while at worst outright cultish.

Not to be confused with

 * The 19th century Zionist movement also called Christian Restorationism.
 * Christian Reconstructionism, a theocratic political movement closely linked to dominionism (the two terms are sometimes used interchangeably).
 * Polytheistic reconstructionism, which has nothing to do with Christianity in any sense.