Since the end of the Cold War, religion has re-emerged as a key instrument in the foreign policies of revisionist states. In some Orthodox contexts, the Byzantine legacy remains tangible in statecraft, where the distinction between Church and State often blurs. Following Samuel P. Huntington's scheme on "civilizational poles", the paper examines how the Russian Orthodox Church (ROC) and the Serbian Orthodox Church (SOC) have contributed to the sacralization of geopolitical projects through the intertwined notions of the Russian World (Russkiy Mir) and the Serbian World (Srpski Svet). It explores how religious narratives and symbols have been mobilized to legitimize external interventions, redefine civilizational boundaries, and sustain national identity discourses. The paper asks how the ROC and SOC frame and justify the political and territorial ambitions of Moscow and Belgrade in theological terms, to what extent they act as autonomous norm entrepreneurs or instruments of state power, and what similarities and divergences mark their use of religion as a foreign policy tool. Methodologically, the analysis combines discourse analysis of ecclesiastical statements and religious diplomacy (2014-2024) with process tracing of Church-state interaction, drawing on comparative area studies and literature on religion and soft power. Preliminary findings suggest that both Churches contribute to the hybridization of religion and geopolitics, blurring the boundaries between spiritual mission and statecraft. While the ROC operates as a transnational ideological arm of Moscow's foreign policy, particularly in justifying the war in Ukraine and supporting Russian-led post-Soviet integration, the SOC functions as a semi-autonomous legitimizing force for Belgrade's regional ambitions in the Western Balkans, specifically in Bosnia-Herzegovina, Montenegro, Kosovo, and North Macedonia. Together, they reveal how religion can legitimize and advance contemporary geopolitical revisionism.