The Catholic Bishop of Lugdunum in Gaul , Irenaeus, should be studied first before thinking his writings are unbiased.
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Against Heresies also speaks against Marcion, who taught that the God of the Old Testament was different from the God of the New Testament and who rejected the Old Testament. Irenaeus’ critique of Gnosticism was largely dependent upon the Old Testament, so he addressed the Marcion heresy as well.
In
Against Heresies, Irenaeus also quotes from every New Testament book except 3 John, so it is an important document to show the ancient church’s acceptance of New Testament Scripture. However,
Against Heresiesalso refers to
The Shepherd of Hermas as “Scripture” and treats the book of 1 Clement as authoritative. Still, in Irenaeus we can see that the canon was beginning to coalesce.
Irenaeus’
The Proof of Apostolic Preaching (or
The Demonstration of the Apostolic Preaching) is a summary of Christian preaching with an emphasis on the fact the Christ fulfills Old Testament prophecy. "