Irenaeus wrote in Greek many works which have secured for him an exceptional place in Christian literature, because in controverted religious questions of capital importance they exhibit the testimony of a contemporary of the heroic age of the Church, of one who had heard St. Polycarp, the disciple of St. John, and who, in a manner, belonged to the Apostolic Age. This book is a treatise in five books, commonly entitled Adversus haereses, and devoted, according to its true title, to the "Detection and Overthrow of the False Knowledge". It is the chief work of Irenaeus and truly of the highest importance; it contains a profound exposition not only of Gnosticism under its different forms, but also of the principal heresies which had sprung up in the various Christian communities, and thus constitutes an invaluable source of information on the most ancient ecclesiastical literature from its beginnings to the end of the second century. In refuting the heterodox systems Irenaeus often opposes to them the true doctrine of the Church, and in this way furnishes positive and very early evidence of high importance. Suffice it to mention the passages, so often and so fully commented upon by theologians and polemical writers, concerning the origin of the Gospel according to St. John, the Holy Eucharist, and the primacy of the Roman Church.