Very little contemporary evidence for the diversity of Christian paschal practice that preceded the Council of Niceae has survived. A unique exception, however, is the paschal tract of Anatolius, bishop of Laodicea in modern Syria, written at least four decades before the Council. In this he vigorously expounded his own views on the specifically ‘Catholic’ celebration of the pasch, and supplied considerable detail regarding other paschal traditions with which he disagreed. A full Latin translation of Anatolius has survived in the text De ratione paschali, whose content, however, is technically complex, blending Biblical exegesis, calendrical science, and astronomy. As a result, the work has been generally neglected in modern times and no serious effort made to try to understand its significance. This book presents the first critical edition of the text and provides the first modern study intended to penetrate the meaning of the whole text, and to make it available to a modern reader.