This paper examines Russo-Ukrainian relations from the mid-seventeenth to the late eighteenth century through the lens of Orthodox identity. It argues that Orthodoxy functioned as a shared framework of legitimation while also generating persistent conflict over authority and belonging. During the Cossack revolution, political claims were articulated in confessional terms: texts speak of "our Ruthenian/Christian people," ground this community in the "ancient Greek faith," and place church affairs among the central demands addressed to the Commonwealth. In 1654 appeals to a common faith were instrumental in mobilising the Muscovite tsar as protector, while the same discourse asserted the antiquity and priority of the Kyivan tradition, traced to Volodymyr's baptism and to Kyiv as a foundational centre of Rus' Christianity. The paper highlights an asymmetry in this confessional language. Ukrainian actors tended to construct kinship primarily in a vertical relationship with the Orthodox monarch, rather than as horizontal solidarity with a "Russian people." After 1654, as disputes accumulated over rights, property and jurisdiction, Orthodoxy increasingly served as a medium for boundary-making. The subordination of the Kyivan metropolitanate intensified this dynamic by converting "protection" into hierarchy. In episodes of open confrontation, confessional rhetoric supported accusations of disloyalty and betrayal within an otherwise shared Orthodox space. The paper therefore explains how a common confession produced both integration and rupture in early modern Russo-Ukrainian relations and shaped later contests over Orthodox unity.