In the field of comparative legal history, Ethiopia is still an unknown country. One of its treasures is the Fetha Nagast, a book of law which had a great influence in the history of Ethiopia and still has great consideration in the society, with its richness in Biblical and Christian principles. This book presents for the first time an ecclesiological and missiological reflection on the Fetha Nagast. The first part of the work is focused on the origin, structure and content of this book of law. In the second part, the author presents the ecclesiology of the Fetha Nagast and its implications and prospectives in Ethiopian Catholic Church. Other aspects studied are brotherhood ecclesiology, the role of the Holy Spirit in the past and present and the notion of Church in the Fetha Nagast, as well as the history of Christianity in Ethiopia.