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.
Gnosticism focuses on special knowledge only available to selected persons (Payton, Christian Faith, 6). Until finding the Gnostic manuscripts in Nag Hammadi (1945), most of scholared information of Gnosticism was found in Against Heresies. They believe the spiritual insights of the world were privy to a select few, turning to the value of immaterial order. This immaterial is spiritual; material is non-spiritual.
Gnosticism put spiritual weight on the immaterial order. The "God" of the Old Testament, later called the Demiurge, is the creator, which was defect. Furthermore, Christ could have bore the nature of a physical body, rather he "seemed to posses" a body. The human nature descended upon Jesus at Baptism and left him prior to his crucifixion. Jesus did not need to suffer in the body because "nothing material can partake of salvation" (Payton, Christian Faith, 12; cf. Against Heresies 1.6.1–2; 1.24.5; 1.27.3; 4.pref.4; 5.2.2–3; 5.6.2).
Against Heresies provides a robust biblical theology, answering the claims of Gnosticism. God is a coherent unity. Creation is good. He provides comments on the unity of the testaments. He provides extensive discussion on the nature of the Old and New Covenant, describing their similarities and dissimilarities. He provides comments on theology proper, soteriology, and eschatology. He helps pave the way for the forthcoming christological debates of the human and divine nature of Christ. Irenaeus provides an early, robust biblical theology of the Christian faith that is pertinent for today's theological construction.
The majority of information about Gnosticism is taken from James R. Payton Jr., Irenaeus on the Christian Faith: A Condensation of Against Heresies (Eugene, OR: Pickwick Publications, 2011), ix–xiv, 1–26.
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I'm always wanting to further and help the discussion on thinking and critical interaction. Here are some research questions I had when reading Against Heresies.
1. Paul is attributed authorship to Ephesians (5.2.3), 1–2 Thessalonians, Galatians, 1–2 Corinthians, Romans, and Revelation are mentioned my name, all four gospels are mentioned as authentic gospels with their designated authors (3.1.1), and the Shepherd of Hermas is called scripture (4.20.2). What is the state of the Canon when Irenaeus writes Against Heresies?
2. How does Irenaeus develop, what we now call, theology proper? (cf. 4.20.6)
3. Irenaeus spends extensive time discussion the Trinity. How does Irenaeus define an Old Testament Christology?
4. How does the broader early church approach the Hebrew OT and the LXX? According to 3.21.2, Irenaeus states "...the Hebrew Scriptures had been translated into Greek by the inspiration of God..."
5. What the similar and dissimilar qualities of the Old and New Covenant according to Irenaeus?