Much scholarly research has been devoted to the question: What issues divide gnostic Christians from the “ecclesiastical” Christians who are their contemporaries? Irenaeus, bishop of Lyons (c. 160 A.D.), whose writings constitute a major source for the investigation of gnosticism, defines the issues in terms of differences in doctrine. Irenaeus admits that the question is a difficult one, since the gnostic doctrines he intends to “expose and refute” are so similar to orthodox teaching that, he says, most Christians cannot differentiate between the two. Irenaeus insists, nevertheless, that the differences are crucial, since gnostic doctrines are false, spurious forms of genuine Christian teaching. Therefore he writes five volumes—The Exposure and Overthrow of Falsely-So-Called Gnosis—to help his fellow churchmen make this discrimination.