The imperial church of the late 2nd century was bedevilled by external competitors -- Gnostics, Marcionites, Montanists – and vexed by internal division over the nature of Christ. Was he man, god, or both? The church brought forth fighters to defend its corporate markets. These were the heresy hunters. Justin Martyr and Hegesippus the Holy were early soldiers. 

Bishop Irenaeus of Lyon was the greatest of these warriors. His works was enormously influential. For a start, he decisively moved the church away from its reliance on the Jewish holy books as divine authorities, and towards a new holy canon. 

In his greatest work, “Against Heresies”, Irenaeus produced an encyclopedia of the church’s enemies. He invented the concept of heresy, incorrect belief. This was a concept unknown to the ancient world. Irenaeus used the concept to set up clear borders between the church incorporate and its rivals.