From Book I, chap. 1 of his monumental Against Heresies, as found in ANF, p. 315:
“Inasmuch as certain men have set the truth aside, and bring in lying words and vain genealogies, which, as the apostle says, minister questions rather than godly edifying which is in faith, and by means of their craftily-constructed plausibilities draw away the minds of the inexperienced and take them captive, [I have felt constrained, my dear friend, to compose the following treatise in order to expose and counteract their machinations.] These men falsify the oracles of God, and prove themselves evil interpreters of the good word of revelation. They also overthrow the faith of many, by drawing them away, under a pretence of [superior] knowledge, from Him who rounded and adorned the universe; as if, forsooth, they had something more excellent and sublime to reveal, than that God who created the heaven and the earth, and all things that are therein. By means of specious and plausible words, they cunningly allure the simple-minded to inquire into their system; but they nevertheless clumsily destroy them, while they initiate them into their blasphemous and impious opinions respecting the Demiurge; and these simple ones are unable, even in such a matter, to distinguish falsehood from truth.
Error, indeed, is never set forth in its naked deformity, lest, being thus exposed, it should at once be detected. But it is craftily decked out in an attractive dress, so as, by its outward form, to make it appear to the inexperienced (ridiculous as the expression may seem) more true than the truth itself. One far superior to me has well said, in reference to this point, A clever imitation in glass casts contempt, as it were, on that precious jewel the emerald (which is most highly esteemed by some), unless it come under the eye of one able to test and expose the counterfeit. Or, again, what inexperienced person can with ease detect the presence of brass when it has been mixed up with silver? Lest, therefore, through my neglect, some should be carried off, even as sheep are by wolves, while they perceive not the true character of these men,— because they outwardly are covered with sheep’s clothing (against whom the Lord has enjoined Matthew 7:15 us to be on our guard), and because their language resembles ours, while their sentiments are very different—I have deemed it my duty (after reading some of the Commentaries, as they call them, of the disciples of Valentinus, and after making myself acquainted with their tenets through personal intercourse with some of them) to unfold to you, my friend, these portentous and profound mysteries, which do not fall within the range of every intellect, because all have not sufficiently purged their brains. I do this, in order that you, obtaining an acquaintance with these things, may in turn explain them to all those with whom you are connected, and exhort them to avoid such an abyss of madness and of blasphemy against Christ. I intend, then, to the best of my ability, with brevity and clearness to set forth the opinions of those who are now promulgating heresy.”