Against Heresies, or On the Detection and Overthrow of the So-Called Gnosis (Ancient Greek : Ἔλεγχος καὶ ἀνατροπὴ τῆς ψευδωνύμου γνώσεως), sometimes referred to by its Latin title Adversus Haereses, is a work of Christian theology written in Greek about the year 180 by Irenaeus, the bishop of Lugdunum (now Lyon in France).
Until the discovery of the Library of Nag Hammadi in 1945, Against Heresies was the best surviving contemporary description of Gnosticism. Today, the treatise remains historically important as one of the first unambiguous attestations of the canonical gospel texts and some of the Pauline epistles. Irenaeus cites from most of the New Testament canon, as well as the noncanonical works 1 Clement and The Shepherd of Hermas; however, he makes no references to Philemon, 2 Peter, 3 John or Jude– four of the shortest epistles.
Against Heresies can be dated to sometime between 174 and 189 CE, as the list of the Bishops of Rome includes Eleutherius, but not his successor Victor. The earliest manuscript fragment of Against Heresies, P. Oxy. 405, dates to around 200 CE.
Irenaeus' primary goal in writing Against Heresies was to attack rival Christian sects that he deemed heretical, mainly the Gnostics and Marcionites.  In particular, he sought to disprove what he saw as incorrect interpretations of scripture on the part of Gnostics such as Valentinus. Irenaeus sought to present "what was understood as an authentic form of century-old Christian tradition against various forms of Gnosticism."
As bishop, Irenaeus felt compelled to keep a close eye on the Valentinians and to safeguard the church from them. In order to fulfill this duty, Irenaeus became well informed of Gnostic doctrines and traditions. His studies of Gnosticism eventually led to the compilation of this treatise.
Irenaeus argued that his conception of orthodox Christianity was passed down to him from the apostles who knew Jesus personally, while the Gnostics and Marcionites were distorting this apostolic tradition.
Irenaeus maintained that human salvation has two components: first, humans must make an intentional commitment to goodness; they then become immortal through the divine power of resurrection. While the Gnostics offered salvation through secret knowledge available only to a few, Irenaeus contended that the true doctrines of the Christian faith are the same taught by bishops in different areas.
While many of the Gnostics viewed the material world as flawed and from which believers sought to escape to an eternal realm of spirit, Irenaeus saw creation as good and ultimately destined for glorification. As Mark Jeffrey Olson points out, 1 Corinthians 15:50 is quoted more than any other verse from the letters of Paul in Against Heresies:
I tell you this, brethren: flesh and blood cannot inherit the kingdom of God, nor does the perishable inherit the imperishable.— 1 Corinthians 15:50, RSV
Both Irenaeus and the Valentinians use this verse to argue for their own understandings of the resurrection of the dead. The Valentinians believed that resurrection was a purely spiritual phenomenon, while Irenaeus insisted that Christians would be raised from the dead in fleshly bodies. According to Irenaeus, this verse was used by the Gnostics to argue that "the handiwork of God is not saved."
Irenaeus also polemicized against Marcion of Sinope, who preached that the creator God of the Hebrew Bible and the Father of Jesus Christ were two different gods. Irenaeus argues that the same god who sent Jesus to the Earth also led man through history by way of the Jewish law and prophets.
- Book 1: I. Valentinus, II. the Propator, III. the misuse of the Bible, IV. the mother Achamoth, V. the Demiurge, VI. The threefold man, VII. against the incarnation, VIII. the Valentinians misuse of the Bible, IX. refutation by Irenaeus, X. the unity of the church, XI. Valentinus' disciples and others. XII. Ptolemy (gnostic) and Colorbasus. XIII. Marcus (Marcosian). XIV. letters and syllables. XV. Sige on the twenty-four elements. XVI. the Marcosians. XVII. Marcosians. XVIII. Misuse of passages from Genesis. XIX. misuse of Bible XX. apocryphal scriptures, XXI. the heretics on redemption, XXII. deviations from the truth. XXIII. Simon Magus and Menander. XXIV. Saturninus and Basilides. XXV. Carpocrates. XXVI. Cerinthus, the Ebionites, and Nicolaitans. XXVII. Cerdo and Marcion. XXVIII. Tatian, the Encratites. XXIX. Borborians. XXX. Ophites and Sethians. XXXI. Cainites and conclusion of Book I.
- Book 2: A rebuttal of the Gnostic systems employing philosophical arguments primarily rather than employing Scripture.
- Book 3: Rebuttal based on apostolic succession and tradition passed down of the faith; defense of the incarnation of Jesus; defense of the virgin birth.
- Book 4: Demonstration that the God of the Old Testament is the God of the New Testament.
- Book 5: A defense of the physical resurrection and eternal judgement.
- Also called On the Detection and Overthrow of Knowledge Falsely So Called e.g. Peter Drilling Premodern faith in a postmodern culture 2006 p73 "But eventually The Detection and Overthrow of Knowledge Falsely So-Called (the actual title of what is commonly known as Against Heresies) expanded from two volumes to five."
- Robert Lee Williams Bishop lists 2005 p123 "Irenaeus recorded the bishops of the Roman church in the third of his five books entitled Detection and Overthrow of Knowledge Falsely So-Called" (Greek: Ἔλεγχος καὶ ἀνατροπὴ τῆς ψευδωνύμου γνώσεως, lit. "Elenchus and Overturning of the Pseudonymous Knowledge"), commonly called Against Heresies (Latin: Adversus haereses, Greek: Κατὰ αἱρέσεων).
- The final phrase "of knowledge falsely so-called" (Greek: tes pseudonymou gnoseos genitive case; or nominative case pseudonymos gnosis)."Greek Word Study Tool | irregular nom f. sg". perseus.tufts.edu. Retrieved 30 March 2016.
- Unger, Dominic J., Dillon, John J., St. Irenaeus of Lyons Against the heresies, Vol.1, p.3, 1992 "the final phrase of the title "knowledge falsely so-called" is found in 1 Timothy 6:20”.
- Due to its reference to Eleutherus as the current bishop of Rome, the work is usually dated c. 180. Schaff, Philip (2001) [c. 1885] "Introductory Note to Irenæus Against Heresies", Ante-Nicene Fathers, Volume I, Against Heresies, William B. Eerdmans Publishing Company.
- Only fragments of the original Greek text exist, but a complete copy exists in a wooden Latin translation, made shortly after its publication in Greek, and Books IV and V are also present in a literal Armenian translation.Poncelet, Albert (1910). "St. Irenaeus". Catholic Encyclopedia. New York City: Robert Appleton Company. Retrieved 4 March 2009.
- Davis, Glenn (2008). "Irenaeus of Lyons". The Development of the Canon of the New Testament. Retrieved 4 March 2009.
- Richardson, C. (1995). Early Christian Fathers. Touchstone. p. 343. ISBN 9780684829517. Retrieved 30 March 2016.
- Egypt Exploration Fund (1903). Grenfell, Bernard P.; Hunt, Arthur S. (eds.). The Oxyrhynchus Papyri. 3. Oxford. p. 10.
405 consists of seven fragments written in a small neat uncial hand, which is not later than the first half of the third century, and might be as old as the later part of the second.
- Heide, G. (2012). Timeless Truth in the Hands of History: A Short History of System in Theology. Wipf & Stock Publishers. p. 15. ISBN 9781630877989. Retrieved 30 March 2016.
- "Donovan, Mary Ann. "Irenaeus of Lyons (review)", Journal of Early Christian Studies 6.4 (1998) 674-675". muse.jhu.edu. Retrieved 30 March 2016.
- Great Christian Thinkers: From the Early Church Through the Middle Ages. Fortress Press. 2011. p. 12. ISBN 9780800698515. Retrieved 30 March 2016.
- Steenberg, M.C. (2009). Of God and Man: Theology as Anthropology from Irenaeus to Athanasius. Bloomsbury Publishing. p. 21. ISBN 9780567600479. Retrieved 30 March 2016.
- Anderson, W.P. A Journey Through Christian Theology: With Texts from the First to the Twenty-first Century. Fortress Press. p. 18. ISBN 9781451420326. Retrieved 30 March 2016.
- Vallée, Gérard (1981). A study in anti-Gnostic polemics: Irenaeus, Hippolytus, and Epiphanius. Waterloo, Ontario: Wilfrid Laurier University Press. p. 9. ISBN 0-919812-14-7. OCLC 8975860.
- Freeman, Jennifer. "Irenaeus, Against Heresies", Christian History, issue 116, 2015 a, /
- Kotsko, A. (2010). The Politics of Redemption: The Social Logic of Salvation. Bloomsbury Publishing. p. 71. ISBN 9780567204325. Retrieved 30 March 2016.
- McFarland, I.A. (2009). Creation and Humanity: The Sources of Christian Theology. Westminster John Knox Press. ISBN 9780664231354. Retrieved 30 March 2016.
- Irenaeus (2001) [c. 180] "Showing how that passage of the apostle which the heretics pervert, should be understood; viz., 'Flesh and blood shall not possess the kingdom of God.'", in Philip Schaff, Ante-Nicene Fathers, Volume I, Against Heresies, Book V, Chapter IX, William B. Eerdmans Publishing Company.
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- Against Heresies – full text at New Advent
- Against Heresies – full text at Christian Classics Ethereal Library
- Adversus Haereses — full text in Latin (and Greek where it exists)
- Against Heresies public domain audiobook at LibriVox