o Tertullian’s writing established a technical theological lexicon
for later writers.• In Gaul, Irenaeus (c. 130–c. 200) was of even greater significance
as a shaper of an orthodox tradition based in tradition and reason.
o Probably born in Smyrna, as a young boy, Irenaeus had met the
martyr Polycarp. He became a presbyter in Lyons, an important
Roman city in Gaul. After the martyrdom of Pothinus (c. 178),
in the persecution under Marcus Aurelius, he became bishop
of Lyons.o Irenaeus was a conciliatory voice in relations with the
Montanists and, later, with the Quartodecimans of Asia Minor.
He resisted the effort to excommunicate them.o He wrote an apologetic work (Demonstration of the Apostolic
Preaching), but his masterpiece was Adversus omnes haereses
(Against All Heresies), written in Greek in five books.• Despite differences in location, language, and temperament,
Tertullian and Irenaeus—and many others—shared the conviction
that there was a “truth” to Christianity that was not a matter of
personal or individual experience but a matter of communal
teaching (orthodoxy) and moral behavior (orthopraxy). For them,
Christianity was inherently social and institutional in character.A Threefold Definition of Christianity
• In Irenaeus’s Against Heresies, we can see the basic instruments
that were deployed in the effort to define Christianity in terms
of orthodoxy.
• Irenaeus himself conceived of Christianity in terms of Greco-
Roman philosophy and perceived and described other versions
of Christianity in the same terms. Thus, he describes “heresies”
(“parties”) in terms of philosophical schools, divided by
doctrinal differences.