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The relationship between the Catholic Church and various Christian denominations in Indonesia, especially North Sumatra, continues to develop in an effort to build unity among the people. A long history of doctrinal differences and the influence of colonial missions have shaped the complex ecumenical landscape in this region. Nevertheless, the spirit of the Council of Nicea, which affirmed the common foundation of Christian faith, remains an inspiration for interdenominational dialog and cooperation. This article examines how the legacy of the Council of Nicea contributes to ecumenism in North Sumatra through a local wisdom-based approach with the principles of marsiadapari or mutual cooperation within the community and dalihan na tolu or the kinship system that governs social relations in Batak society. These cultural values serve as a bridge in an effort to overcome theological differences and seek to strengthen inter-church solidarity. Using a qualitative method through a historical theological approach, this research analyzes various ecumenical initiatives involving the Catholic Church and other Christian denominations. These initiatives take the form of pastoral care, participation in joint liturgical celebrations, and involvement in culturally rooted church deliberations. The findings show that in the spirit of the Council of Nicea and collaboration with local cultural values can foster a more inclusive and sustainable Christian communion. By using local wisdom as a foundation for dialog, the Catholic Church and Christian communities in North Sumatra can strengthen their role in a multicultural society. This study confirms that culture-based ecumenism offers a powerful model for interdenominational cooperation, thus strengthening Christian faith and mission in Indonesia.

Ecumenism; Nicene Council; Catholicism; Christianity; North Sumatra; local wisdom; dialogue