On what grounds could Aquinas's interpretation of Isaiah be called mystagogical? How does he account for growth in faith? What is Aquinas's conception of teaching? What does he say on the mystagogy of the Holy Spirit? Can Aquinas help us understand the concept of sensus fidei? What is the relationship between Aquinas and Pseudo-Dionysius concerning mystical theology? How about baptism? How about moral transformation? What can we say about the preaching of Aquinas? These and other questions were discussed at the sixth international conference of the Thomas Instituut te Utrecht (Tilburg University), held in December 2018 in Utrecht. The conference was devoted to the theme of Initiation and Mystagogy in Thomas Aquinas, a surprising and challenging theme. A number of studies presented at this conference are gathered together here in this volume. The essays address a wide array of subjects and attest to a variety of perspectives, because of which the volume is divided into four parts: (I) Scriptural, (II) Systematic, (III) Sacramental and Moral, and (IV) Pastoral perspectives.