The article discusses the ideas of Mikhail V. Zyzykin (1880–1960) about the contribution of the Church to international law in the context of its history and the international relations in the 1930s. Special attention is paid to the relation of Orthodoxy to international law, since Zyzykin is one of the few jurists who have studied in detail the influence of the Orthodox Church tradition on the law of nations. His works on this subject (first of all, an essay The Church and International Law (1937), based on a report at the Oxford conference of practical Christianity in 1937), remain little known to social and political science. The article considers the main provisions of Zyzykin about the origin of international law in medieval Europe with the participation of the Church in the context of the positions of other international lawyers (Taube, Martens, Kamarovsky, Nys, Bluntschli). It contains a comparative characteristic of the attitude to international law of the three Christian denominations (Catholicism, Protestantism, and Orthodoxy) according to Zyzykin and his idea of the Christian West and East “asymmetric” international contribution (the East was represented initially by the Eastern Roman Empire, and then by the Russian Empire). The fourth part describes the most original part of Zyzykin’s ideological legacy: a comparison of two vulnerable attempts of international organization, the Holy Alliance in the XIX century and the League of Nations in the XX century.