The Catacomb Church () as a collective name labels those representatives of the Russian Orthodox clergy, laity, communities, monasteries, brotherhoods, etc., who for various reasons, have moved to an illegal position since the 1920s. In a narrow sense, the term "catacomb church" means not just illegal communities, but communities that rejected subordination to the Acting patriarchal locum tenens Metropolitan Sergius (Stragorodsky) after 1927, and adopted anti-Soviet positions. During the Cold War the ROCOR popularized the term in the latter sense, first within the Russian diaspora, and then in the USSR by sending illegal literature there. The expression "True Orthodox church" () is synonym for this latter, narrower sense of "catacomb church". The historian argues that "the catacombness of the Church does not necessarily mean its intransigence. This term covers all unofficial and therefore not state-controlled church activities". Organizationally, the Catacomb Church communities were usually not interconnected. Origin The death of Patriarch Tikhon, the leader of the Russian Orthodox Church in April 1925 led to unrest among the followers of the church. Tikhon's designated successors were arrested by the civil authorities. Metropolitan Sergius became the Acting patriarchal locum tenens of the Moscow Patriarchate. Sergius issued calling all members of the Russian Orthodox Church to profess loyalty towards the Soviet government. The declaration sparked division among the hierarchy, clergy, and laity, which led to the formation of the Russian True Orthodox Church, or Catacomb Church, a group of which was the Josephite movement. Opposition to Sergius' declaration was based not only on his political concessions, but also on canonical and theological disagreements. Terminology The earliest documented use of the word "catacombs" to describe the Russian realities of the 20th century is found in the letters of abbess Athanasia (Gromeko) to Metropolitan Eulogius (Georgievsky), written in 1923 from Petrograd. After the nuns were expelled from their church building by the Renovationists, the community did not disband, but continued its existence as a convent in a private home. In two of the four surviving letters, abbess Athanasia uses the expressions "my catacombs" and "my secret catacomb church" several times. It can be seen from the context that this is how she designates her house church, contrasting her "catacombs" with the officially functioning church of the Renovationists. The use of the expressions "catacombs" and "catacomb church" in relation to the 1920s–1930s realities showed a certain educational and cultural level of those who used these concepts.