The second-century Church Father Irenaeus's most famous work, Against Heresies, was principally directed at the contemporary heresy of Gnosticism, especially as that movement found expression in the thought of a teacher named Valentinus in Rome. One can discern in Valentinus's doctrine the two chief characteristics of Gnosticism: a strong distaste for the material world (and its captivity to change and decay); and bizarre speculation about how the world came into existence (which speculation generally served to reinforce distaste for the material world). Valentinus held that ultimate spiritual reality comprises a hierarchy of thirty gods (collectively known as the 'Pleroma'), the least of which (named Sophia) became consumed with illicit desire to comprehend her superiors and consequently gave birth (as it were) to a spiritual being called Achamoth. Achamoth, in turn, generated the "god" who created this world.
Unlike his contemporary Marcion who simply rejected portions of Christian Scripture that didn't gel with his heterodox theological vision, Valentinus generally accepted whatever texts were being circulated as Christian Scripture and then tried to convince his hearers that such texts had hidden meaning that revealed his bizarre doctrine -- hidden meaning that others could only discover with the help of him or other select teachers who had inherited knowledge of such by means of a secret tradition stretching back to the Apostles. So, for instance, Valentinus discovered a reference to the Pleroma in the parable of the Laborers in the Field (Matt. 20.1-16), because the various hours at which the master sends out laborers into his field (1, 3, 6, 9, and 11) add up to thirty (the number of gods in the Pleroma). As silly as such a reading of Scripture might seem to us in the present, many people were led astray by such teaching, perhaps because it satisfied an innate human itch to have the one-up on others, to be in the know (or rather, gnosis) about what Scripture really means.
Valentinus's system, and ones similar to it, presented Irenaeus and other apologists of orthodox Christianity with perfect opportunities to reflect upon proper methods for reading and understanding Scripture. Thus Irenaeus advanced in his work the basic thesis that Scripture means what it says -- that is, that Scripture is clear in its articulation of the fundamental points of Christian theology, and that anyone of sound mind can actually pick up Scripture, read it, and grasp those points.
"A sound mind," he wrote, "and one which does not expose its possessor to danger, and is devoted to piety and the love of truth, will eagerly meditate upon those things which God has placed within the power of mankind [i.e., power to understand], and has subjected to our knowledge, and will make advancement in them, rendering the knowledge of them easy to him by means of daily study. These things are such as fall under our observation, and are clearly and unambiguously in express terms set forth in the Sacred Scriptures." (Against Heresies 2.27)
Irenaeus was not naive. No matter how clear and unambiguous Scripture's teaching of Christianity's main points might be, "advancement" in understanding that teaching, to his thinking, still required eagerness, devotion to piety and love of truth, daily study, and meditation. He also recognized (as Peter did [2 Peter 3.14-16]) that Scripture contains some passages that aren't so easy to decipher. Thus he complemented his basic thesis about Scripture's clarity with guidelines for navigating the more difficult texts. He encouraged his readers to read and interpret those more tricky passages through lenses provided both by the clear texts (see Against Heresies 2.28) and the teaching of Scripture as a whole, which teaching can be summarized in creedal form (AH 1.10). He also reminded his readers that "the treasure hid in the Scriptures is Christ" (AH 4.26) -- that is, that one's understanding of a biblical text, difficult or otherwise, should conform and lead to Christ, whose person and work stands at the very center of God's Word to us through the prophets and apostles.
Irenaeus also insisted that one should rely on authorities within the Church whose very job description entails receiving, safeguarding, and passing along the Church's corporate and traditional sense of what constitutes biblical truth. But he was more cautious in this admonition than is sometimes claimed. He recognized that some persons lawfully holding office in the Church might be wolves rather than sheep, and such persons, rather than serving as guardians of the Word, would ultimately be judged by it. "Those, however, who are believed to be presbyters by many, but serve their own lusts, and, do not place the fear of God supreme in their hearts, but conduct themselves with contempt towards others, and are puffed up with the pride of holding the chief seat, and work evil deeds in secret, saying, 'No man sees us,' shall be convicted by the Word, who does not judge after outward appearance, nor looks upon the countenance, but the heart...." (AH 4.26) Christians must, then, exercise some measure of judgment regarding persons in authority; there can be no blind allegiance to the teaching of any man, no matter his credentials.
Against Heresies is a remarkable piece of writing. I reckon it could serve as a textbook on hermeneutics in most confessionally Protestant seminaries without too much qualification. It's certainly unambiguous in its assertion of Scripture's perspicuity (or clarity), that idea about the Bible that the magisterial reformers supposedly dreamed up in the sixteenth century. It's likewise just as nuanced as the magisterial reformers in laying out principles for navigating less than clear biblical texts and relating one's impressions of biblical meaning to traditional interpretation. As such, Irenaeus's work is a powerful reminder that the Reformation was not a reaction against 1500 years of getting it wrong, but a recovery of genuine catholic Christian beliefs and practices (and so a reaction against the perversion of catholic beliefs and practices in the immediately preceding centuries).