This project offers an original contribution to the interpretation of the documents of the Second Vatican Council that constitute the most authoritative doctrinal teaching within the Catholic Church. The chapters in this volume, published during the 60th anniversary of the Council (2022-2025), discuss three types of stumbling blocks: ‘Stumbling Blocks for Ecumenism’, ‘Stumbling Blocks for Interfaith Dialogue,’ and ‘Stumbling Blocks for Church-world Relations’. Eight specialists of ecclesiology, comparative theology, intercultural theology, and theological ethics have each written chapters on a selected line of Vatican II that constitutes a ‘stumbling block’ or ‘hard saying’ for believers and theologians today. The views expressed in these chapters have been discussed in three response essays. The stumbling blocks have been selected from Lumen Gentium, Unitatis Redintegratio, Nostra Aetate, and Gaudium et Spes. The selected lines discuss the difficulties the Catholic Church has with atheism and with the Eucharist as celebrated by Protestant ministers; how appreciation of other churches and religions goes hand in hand with defending the need of mission; and why the Council assigns different roles to priests and laity, making a distinction between the holiness of the Church and the sinfulness of its believers.