You have indeed enjoined upon me, my very dear friend, that I should bring to light the Valentinian doctrines, concealed, as their votaries imagine; that I should exhibit their diversity, and compose a treatise in refutation of them. I therefore have undertaken— showing that they spring from Simon, the father of all heretics— to exhibit both their doctrines and successions, and to set forth arguments against them all. Wherefore, since the conviction of these men and their exposure is in many points but one work, I have sent unto you [certain] books, of which the first comprises the opinions of all these men, and exhibits their customs, and the character of their behaviour.
In the second, again, their perverse teachings are cast down and overthrown, and, such as they really are, laid bare and open to view. But in this, the third book I shall adduce proofs from the Scriptures, so that I may come behind in nothing of what you have enjoined; yea, that over and above what you reckoned upon, you may receive from me the means of combating and vanquishing those who, in whatever manner, are propagating falsehood. For the love of God, being rich and ungrudging, confers upon the suppliant more than he can ask from it.
Call to mind then, the things which I have stated in the two preceding books, and, taking these in connection with them, you shall have from me a very copious refutation of all the heretics; and faithfully and strenuously shall you resist them in defence of the only true and life-giving faith, which the Church has received from the apostles and imparted to her sons. For the Lord of all gave to His apostles the power of the Gospel, through whom also we have known the truth, that is, the doctrine of the Son of God; to whom also did the Lord declare: “He that hears you, hears Me; and he that despises you, despises Me, and Him that sent Me.”
Source: Adversus haereses (New Advent)