The unity of the Church is one of the fundamental issues in Christianity, continuously shaping its mission and ecclesial structures. Since the Council of Nicaea (AD 325), when the Creed was formulated as an expression of common faith, the tension between unity and diversity has become a central challenge for ecclesiology. Contemporary Christianity operates in a context of cultural, theological, and institutional multipolarity, necessitating a new approach to the understanding of unity. In this framework, the concept of polycentricity emerges—a model of the Church in which unity does not mean uniformity but rather cooperation and complementarity among different decision-making centres, both at local and global levels. This paper aims to explore the theological foundations of polycentricity in light of the doctrinal and historical implications of the Nicene Creed. Particular attention will be given to the thought of John Henry Newman regarding doctrinal development and ecclesiology, examining its potential relevance for contemporary ecumenism. The presentation will also consider the practical implications of polycentricity for the relationship between local churches and the Apostolic See, as well as for contemporary ecumenical challenges such as interdenominational dialogue and social justice. In the context of the 1700th anniversary of the Nicene Creed, the discussion will reflect on whether a polycentric model can provide an adequate response to the challenges of contemporary Christianity and how it may contribute to fostering the unity of the Church in the 21st century.