Irenaeus of Lyons (in modern France) was the first great expositor of Christian theology, writing around 175 A.D. Born in Asia Minor (modern Turkey) and taught by the disciples of the apostles, he wrote as a churchman using scripture, his own thoughts, and a few other early writings (Justin Martyr, Papias, Ignatius of Antioch) to explain and defend the deposit of faith he had received. Irenaeus defined his theology in conflict with the Gnostics who frequently shared churches with the Christians but advocated an often wildly divergent theology from that shared by the orthodox Christians. His big book was "Against the Heresies," a five-volume description and refutation of the Gnostic beliefs. He describes the Valentinian Gnostics in detail; the Marcionites and other schools get less attention. Thus Irenaeus can be read both to find out what Gnosticism was like, and also to find out about Christian theology in the second century A.D.
No modern unabridged translation of "Against the Heresies" exists. Dominic Unger translated only the first of the book's five volumes, that which consists of a simple description of Gnostic beliefs without detailed refutation. It is unclear if any subsequent volume will appear. In the meantime, Robert Grant in this book "Irenaeus of Lyons" presents an abridged translation of the whole book including virtually all of the the main passages that touch on important theological issues. Even if the complete translation appears, I think general readers will want to stick with Grant's translation. I have the Unger volume and can testify that Irenaeus unabridged is hard to plough through, partly because the beliefs he is refuting seem so colossally strange and partly because Irenaeus tends to repeat the main points several times. Hence few but hard-core specialists would want to read the whole thing.
In his preface, Grant usefully points out the importance of "hypothesis" (meaning the overall plot line) and "economy" (meaning dispensation or sub-plot, more or less ) in Irenaeus's thinking. The "hypothesis" and "economy," which together make what Irenaeus calls "the Rule of Faith" (basically something like the later Nicene and Apostolic creeds), is the big story of Creation, Fall, and Redemption. To Irenaeus, the problem with the Gnostics is that they broke free from this Rule of Faith in order to answer the puzzles of theology and scripture. Irenaeus insisted that the salvation brought by Christ is a "recapitulation" of the blessed state of Adam and Eve before the fall, not a return to some world of pre-material and pre-Creation "eons" (manifestations of the Godhead). Irenaeus testified that he was taught this Rule of Faith by the martyr Polycarp who had it from the apostle John, and that it is identical to the theology taught by the Roman bishops who likewise traced their teaching back to apostles Peter and Paul. While the Gnostics used their concept of a pre-Creation world of interacting "eons" and a division between merely carnal and truly spiritual Christians to explain scriptural puzzles like the many names of God in the Old Testament, the divine Christ and the human Jesus, faith vs. works, and predestination vs. moral responsibility, Irenaeus demonstrated through Scripture (he knew all four of the Gospels, the letters of Paul, 1 and 2 John, and Revelation) that their explanations could not be accepted as responsible interpretations. (Irenaeus later summarized the "overall plot line," together with refutations of Rabbinic Jewish attacks on Christianity in his "Proof of the Apostolic Preaching").
It's also worth noting the pervasive physicality of Irenaeus's theology. Eucharist is the real body of Christ because otherwise how would our body be redeemed? Likewise, there must be a thousand-year earthly rule of the resurrected saints, otherwise Christ would not be redeeming our bodies, and so on. Indeed at some points he seems to be viewing God as a kind of super-huge body surrounding the cosmos. His explanation of the Trinity defines the persons solely by how they relate to the material world rather than by their internal relations: Jesus is defined as the God the Father's Word that creates all things and the Spirit as God's Wisdom that governs the motions of all things. Later Christian theologians lost interest in Irenaeus, whose work seemed somewhat out of date and his works, originally written in Greek, survived only in obscure Latin and Armenian translations. Fortunately scholarship has revived these fascinating early works.
In sum, this is a very useful edition of an important testimonial to the Christian teaching in the first generations after Christ. To judge by this testimony, the orthodox bishops of the early church had great difficulty plumbing the depths of what Paul, John, and the other New Testament writers wrote. Yet they knew in their gut that the Gnostics explanations had to be wrong. Irenaeus, by holding on to the essential "plot line" (hypothesis) of salvation through Christ's recapitulation of the original unfallen state of physical Creation, began the long process of drawing out the "treasures of wisdom and knowledge" hidden in the New Testament.