Was Jesus divine? Was Jesus human? Could God suffer? Can people save themselves by their own efforts? Do Christian ministers have to be perfect? These and other questions were answered by the early Christian Church so as to rule in certain orthodox beliefs and rule out certain heretical beliefs. Anyone could be a Christian, but a Christian could not believe simply anything. Here, twelve top theologians, all practising Christians, tackle ancient heresies and show why the contemporary Church still needs to know about them. The contributors argue that heresies are never finally defeated but always continue in some form or other as live options for belief. Christians therefore need to remember what these great early heresies were and why they were ruled out, or else risk falling prey to their modern-day manifestations. The essays included here are scholarly but accessible, academic but highly relevant. They show how attractive and plausible heresies are and how the Church has always required intellectual effort, moral courage and political skill to resist them.