1 By transmitting to you, my very dear friend, this fourth book of the work which is [entitled] The Detection and Refutation of False Knowledge, I shall, as I have promised, add weight, by means of the words of the Lord, to what I have already advanced; so that you also, as you have requested, may obtain from me the means of confuting all the heretics everywhere, and not permit them, beaten back at all points, to launch out further into the deep of error, nor to be drowned in the sea of ignorance; but that you, turning them into the haven of the truth, may cause them to attain their salvation.
2. The man, however, who would undertake their conversion, must possess an accurate knowledge of their systems or schemes of doctrine. For it is impossible for any one to heal the sick, if he has no knowledge of the disease of the patients. This was the reason that my predecessors— much superior men to myself, too — were unable, notwithstanding, to refute the Valentinians satisfactorily, because they were ignorant of these men's system; which I have with all care delivered to you in the first book in which I have also shown that their doctrine is a recapitulation of all the heretics. For which reason also, in the second, we have had, as in a mirror, a sight of their entire discomfiture. For they who oppose these men (the Valentinians) by the right method, do [thereby] oppose all who are of an evil mind; and they who overthrow them, do in fact overthrow every kind of heresy.
3. For their system is blasphemous above all [others], since they represent that the Maker and Framer, who is one God, as I have shown, was produced from a defect or apostasy. They utter blasphemy, also, against our Lord, by cutting off and dividing Jesus from Christ, and Christ from the Saviour, and again the Saviour from the Word, and the Word from the Only-begotten. And since they allege that the Creator originated from a defect or apostasy, so have they also taught that Christ and the Holy Spirit were emitted on account of this defect, and that the Saviour was a product of those Æons who were produced from a defect; so that there is nothing but blasphemy to be found among them. In the preceding book, then, the ideas of the apostles as to all these points have been set forth, [to the effect] that not only did they, “who from the beginning were eye-witnesses and ministers of the word” of truth, hold no such opinions, but that they did also preach to us to shun these doctrines, foreseeing by the Spirit those weak-minded persons who should be led astray.
4. For as the serpent beguiled Eve, by promising her what he had not himself, so also do these men, by pretending [to possess] superior knowledge, and [to be acquainted with] ineffable mysteries; and, by promising that admittance which they speak of as taking place within the Pleroma, plunge those that believe them into death, rendering them apostates from Him who made them. And at that time, indeed, the apostate angel, having effected the disobedience of mankind by means of the serpent, imagined that he escaped the notice of the Lord; wherefore God assigned him the form and name [of a serpent]. But now, since the last times are [come upon us], evil is spread abroad among men, which not only renders them apostates, but by many machinations does [the devil] raise up blasphemers against the Creator, namely, by means of all the heretics already mentioned. For all these, although they issue forth from diverse regions, and promulgate different [opinions], do nevertheless concur in the same blasphemous design, wounding [men] unto death, by teaching blasphemy against God our Maker and Supporter, and derogating from the salvation of man. Now man is a mixed organization of soul and flesh, who was formed after the likeness of God, and moulded by His hands, that is, by the Son and Holy Spirit, to whom also He said, “Let Us make man.” This, then, is the aim of him who envies our life, to render men disbelievers in their own salvation, and blasphemous against God the Creator. For whatsoever all the heretics may have advanced with the utmost solemnity, they come to this at last, that they blaspheme the Creator, and disallow the salvation of God's workmanship, which the flesh truly is; on behalf of which I have proved, in a variety of ways, that the Son of God accomplished the whole dispensation [of mercy], and have shown that there is none other called God by the Scriptures except the Father of all, and the Son, and those who possess the adoption.
Source: Adversus haereses (New Advent)