In Eastern Orthodox church history, the Old Believers or Old Ritualists (Russian: старове́ры or старообря́дцы, starovéry or staroobryádtsy) are Eastern Orthodox Christians who maintain the liturgical and ritual practices of the Eastern Orthodox Church as they existed prior to the reforms of Patriarch Nikon of Moscow between 1652 and 1666. Resisting the accommodation of Russian piety to the contemporary forms of Greek Orthodox worship, these Christians were anathematized, together with their ritual, in a Synod of 1666–1667, producing a division in Eastern Europe between the Old Believers and those who followed the state church in its condemnation of the Old Rite.

Russian speakers refer to the schism itself as raskol (Russian: раскол), etymologically indicating a "cleaving-apart".

In 1652, Patriarch Nikon (1605–1681; Patriarch of the Russian Orthodox Church from 1652 to 1658) introduced a number of ritual and textual revisions with the aim of achieving uniformity between the practices of the Russian and Greek Orthodox churches. Nikon, having noticed discrepancies between Russian and Greek rites and texts, ordered an adjustment of the Russian rites to align with the Greek ones of his time. In doing so, according to the Old Believers, Nikon acted without adequate consultation with the clergy and without gathering a council. After the implementation of these revisions, the Church anathematized and suppressed—with the support of Muscovite state power—the prior liturgical rite itself, as well as those who were reluctant to pass to the revised rite.

Those who maintained fidelity to the existing rite endured severe persecutions from the end of the 17th century until the beginning of the 20th century as "Schismatics" (Russian: раскольники raskol'niki). They became known as "Old Ritualists", a name introduced during the reign of Catherine the Great. They continued to call themselves simply "Orthodox Christians".

By the middle of the 17th century, Greek and Russian Church officials, including Patriarch Nikon of Moscow, had noticed discrepancies between contemporary Russian and Greek usages. They reached the conclusion that the Russian Orthodox Church had, as a result of errors of incompetent copyists, developed rites and liturgical books of its own that had significantly deviated from the Greek originals. Thus, the Russian Orthodox Church had become dissonant with the other Orthodox churches. Later research was to vindicate the Muscovite service-books as belonging to a different Greek recension from that which was used by the Greeks at the time of Nikon. The unrevised Muscovite books proved to be older than the current Greek books, which had been revised over the centuries, were newer, and contained innovations.^[1][2] Nikon wanted to have the same rite in the Russian tsardom and majority-ethnic Slavic lands (current territories of Ukraine and Belarus), then part of Polish–Lithuanian Commonwealth, to attract local Orthodox rebels. Their rite was closer to the Greek than that in the Moscow duchy. Nikon did not want to adopt two different rites in the same church.

Supported by Tsar Alexis (r. 1645–1676), Nikon carried out some preliminary liturgical reforms. In 1652, he convened a synod and exhorted the clergy on the need to compare Russian Typikon, Euchologion, and other liturgical books with their Greek counterparts. Monasteries from all over Russia received requests to send examples to Moscow to have them subjected to a comparative analysis. Such a task would have taken many years of conscientious research and could hardly have given an unambiguous result, given the complex development of the Russian liturgical texts over the previous centuries and the lack of textual historiographic techniques at the time.

The locum tenens for Patriarch Pitirim of Moscow convened the 1666 Great Moscow Synod, which brought Patriarch Macarios III Zaim of Antioch, Patriarch Paisius of Alexandria and many bishops to Moscow. Some scholars allege that the visiting patriarchs each received both 20,000 rubles in gold and furs for their participation.^[1] This council officially established the reforms and anathematized not only all those opposing the innovations but the old Russian books and rites themselves as well. As a side-effect of condemning the past of the Russian Orthodox Church and her traditions, the messianic theory depicting Moscow as the Third Rome appeared weaker. Instead of the guardian of Orthodox faith, Moscow seemed an accumulation of serious liturgical mistakes.

Nevertheless, both Patriarch and Tsar wished to carry out their reforms, although their endeavors may have had as much or more political motivation as religious; several authors on this subject point out that Tsar Alexis, encouraged by his military success in the Russo-Polish War (1654–1667) to conquer West Russian provinces and Ukraine, developed ambitions of becoming the liberator of the Orthodox areas which at that time formed part of the Ottoman Empire. They also mention the role of the Near-East patriarchs, who actively supported the idea of the Russian Tsar becoming the liberator of all Orthodox Christians and who suggested that Patriarch Nikon might become the new Patriarch of Constantinople.