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.
^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).
^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.
^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.