Which 2nd-century bishop wrote 'Against Heresies' to combat Gnosticism?

The 2nd-century bishop who wrote “Against Heresies” to combat Gnosticism was Saint Irenaeus.

Irenaeus became bishop of Lugdunum, the Roman city now known as Lyon, probably around the 177 massacre of Christians there. Originally from Smyrna in Asia Minor, he had known Polycarp, who was traditionally connected with the apostolic generation. That background helped shape Irenaeus’s emphasis on public teaching handed down through bishops rather than secret revelations.

Written around 180, “Against Heresies” was formally titled “On the Detection and Overthrow of the So-Called Gnosis.” Its five books describe and challenge Valentinian and other Gnostic systems, which often presented salvation as escape from the material world through secret knowledge. Irenaeus instead defended creation, the unity of Scripture, and the continuity of apostolic tradition.

Readers sometimes confuse Irenaeus with Justin Martyr or Ignatius of Antioch, both important early Christian writers. Neither wrote this particular comprehensive refutation. Irenaeus is also notable for one of the earliest surviving witnesses to the importance of all four canonical Gospels.