The research of the history of the Russian Orthodox Church of the 20th century is largely based on the study of documents of that era drawn up in the church milieu. Since all kinds of church publications were prohibited, the reproduction of documents in private — by copying or retyping — gained ground in Russia of the 1920s and 1930s. These documents played the role of the church press — they introduced the events of church life, expressed beliefs in the rightness of the author or a group of like-minded people, unmasked ideological opponents, and also served to communicate with foreign church figures. The reproduction of such documents, in case of their discovery by authorities, could endanger not only the authors thereof and the persons mentioned therein, but also ordinary copyists and put them all to punitive measures such as purge. Thanks to the ascetics who preserved them, these documents remain the most important information sources in the field of history of the Russian Orthodox Church in the era of persecution. The article lists the documents that were taken abroad — letters of Bishop Damaskin (Cedric), the collection of church documents “The Case of Metropolitan Sergius”. Moreover, it analyzes documents both originating from the clerical office of Metropolitan Sergius (Stragorodsky) and generated by church figures opposed to Deputy Patriarchal Locum Tenens. A large number of documents was drawn up by Mikhail Novosyolov himself and the circle of his close associates. Also, the most important documents of that era are the letters of Metropolitan Kirill (Smirnov) and replies of Metropolitan Sergius (Stragorodsky). The article pays special attention to the transfer abroad of the collection of church documents entitled “The Case of Metropolitan Sergius”, the role of the journalist Mikhail Brоndsted (pseudonym: Mikhail Artemyev), who left abroad in 1930, and his articles published abroad on underground literature in the Soviet Union. The problem of the authorship of anonymous sources, the authenticity of documents distributed in the church milieu is also raised here. The Joint State Political Directorate of the Soviet Union actively used church documents found during arrests to persecute believers. Fragments of these documents often became the basis for indictments, as evidence of the accused’s anti-Soviet activity. The article also mentions the role of collectors of church documents during the persecution against the Church — Archpresbyter Michael Polsky, Mikhail Gubonin, Metropolitan Manuel (Lemeshevsky).