This introductory chapter examines the central theme of this volume: the evolving relationship between post-secularization and diversity in contemporary societies. Anchored in an interdisciplinary dialogue bridging sociology, migration studies, and religious studies, it integrates key theoretical perspectives and empirical insights to contextualize these themes, which run throughout the volume, in relation to globalization, digital transformation, and shifting socio-political dynamics. The chapter revisits the enduring relevance of religion in ostensibly secular societies while addressing the challenges and opportunities posed by pluralism in governance, identity formation, and cultural negotiation. By connecting the theoretical discourse on post-secularization with its practical implications for governing diversity, it lays the groundwork for the volume’s comprehensive exploration of religious expressions, policymaking, and global cultural shifts. Finally, the chapter highlights how the volume’s three sections—Foundations, Governance, and Challenges—work cohesively to address the profound transformations in religion and diversity across both local and global contexts.