This research presents the medieval liturgical exposition of William Durand (1230-1296) Rationale divinorum officiorum as an example of pre-treatise ecclesiology. Medieval authors compiled the first complete theological treatises on the Church (de Ecclesia) after the publication of the bull Unam sanctam (11/18/1302) by Pope Boniface VIII. This bull comes at the end of a historical period that transmits the thoughts about the Church within various theological and liturgical texts. One of the goals of this work is to present theological movements at the turn of the 13th to the 14th century, emphasizing ecclesiological awareness. Before the appearance of the bull mentioned above by Pope Boniface VIII. various liturgical commentaries were prominent in conveying awareness of the Church, among other texts that convey ecclesiological awareness. Therefore, this dissertation presents the ecclesiological elements written within Durand's liturgical exposition. This dissertation has three more extensive chapters, the third of which is central. The first chapter briefly overviews the most essential theological emphases of the developed Middle Ages (1000-1300). With this chapter, the author places the researched source in the broader context of medieval theology, as well as the reform attempts of the Church in the developed Middle Ages. This chapter provides an overview of the state of research in Durand's liturgical exposit, the Croatian nomenclature for the researched type of medieval theological writing, and the research methodology and theses. In the second chapter, the author meticulously presents the research material. The entire Rationale divinorum officiorum was carefully extracted using research questions, each asked with an ecclesiological key. The research material, consisting of the Prologue and the entire First and Second books of Durand's liturgical exposition, was selected for its relevance and depth. Fragments from the Third, Fourth, Fifth and Sixth Books were also included, while the last two books were omitted due to the extensiveness of Durand's liturgical exposition and the specific topics he addresses in those parts of the treatise. The third chapter contains the theological analysis of the research material presented in the previous chapter. In Durand's liturgical exposition, the author first presents the New Testament and patristic awareness of the Church to recognize the elements of the New Testament and patristic ecclesiology. After a theological analysis of the elements of the New Testament and patristic ecclesiology transmitted within the Rationale divinorum officiorum, the author researched the influence of the reform attempts of the 13th-century Church on Durand's text. In the research of the ecclesiological elements found in the Rationale divinorum officiorum, the author uses analysis, description, and synthesis methods. This dissertation aims to present William Durand's Rationale divinorum officiorum as a pre-treatise way of transmitting ecclesiological thought in medieval times. The goal is also to present this medieval author and his liturgical treatise since research on liturgical expository literature has not started in the Croatian-speaking area. The scientific contribution of this dissertation is a more evident knowledge of the historical and theological movements at the turn of the 13th and 14th centuries. This research showed that Durand's liturgical exposition is an example of historical, allegorical, tropological, and anagogic exegesis of biblical texts. Interpreting the elements of worship, Durand brings into his work ecclesiological elements that range from the foundation and essence of the Church to the authority of individual church service, that is, the rights and obligations of individual members of the Church. Apart from the Prologue and the First Book of Durand's liturgical exposition, the rest represents a specific example of pre-treatise ecclesiological awareness. Durand incorporates ecclesiological elements into the Rationale divinorum officiorum using canon law sources to compile his text. Thus, he is not an original author. However, with the help of canon law texts, he realizes spiritual-liturgical ecclesiology, in contrast to later canonical-legal ecclesiology. Durand transferred the biblical and patristic understanding of the Church to medieval thought in his efforts to reform the Church. The way to reform the Church, according to the ideal of the early Church (ecclesia primitiva), Durand seeks to explain everything that the clergy and laity do within the church building and that they live as a community of believers.