Few fathers of the Christian faith appear after the death of John that have extant literature. Very little is left of Papias or Polycarp; some of which exist as quotes in other early literature. Iranaeus may be one of the first richest and earliest theologian in the proceeding decades after John.
Irenaeus (130–202), in Against Heresies, set forth a deafening blow to Gnostic Christianity. Calling it heresy, Against Heresies is a massive 5-volume tome. Half (Book 1–2) is devoted to critiquing Gnosticism and the other half (Book 3–5) is devoted to describing the Christian faith.