The paper would examine the decision of the Holy See not to join the "Board of Peace" initiative proposed by the Trump administration. While the invitation reflects the Holy See's recognized prestige within international diplomacy, the refusal illustrates how this peculiar actor evaluates political initiatives framed around peace and mediation. The Holy See's decision can be properly understood by considering the interaction of theological-doctrinal and political criteria. The Holy See's diplomatic practice is guided by Catholic Social Teaching principles, including the promotion of international law and of a just international order, while also safeguarding its credibility and institutional role as a hybrid actor between international and world society. The refusal to join the "Board of Peace" should therefore be read without reducing it to a contingent political distancing from the current U.S. presidency , even if it reflects potential drivers of friction between American Catholicism and the Trump administration. Overall, this case demonstrates how the Holy See participates in the political dimension of international society as a distinctive religious actor, applying a combination of theological, doctrinal, and prudential criteria within an Augustinian-inspired Christian realism, and illustrates concretely how its theological identity shapes its unique engagement in international politics.