In 1983, the Ecumenical Institute of Bossey (Switzerland) organized a seminar on Roman Catholicism in order to present it to theologians and pastors of other Christian denomination. Bruno Forte was then asked to offer the theological thinking of the Church about herself in light of the Second Vatican Council. This is the origin of these lessons, which constitute a "brief ecclesiology," structured around three main issues: Where does the Church come from? What is the Church? Where does the Church go? These pages were readily accepted in the ecumenical meeting at Bossey, and therefore they are a help to anyone whether Catholic or not who wishes to deepen in the knowledge of the ecclesiology of those churches in communion with Rome. The epilogue, written for this new edition, reviews the validity of his approach and the new challenges faced by the Church.