1 Since therefore we have such proofs, it is not necessary to seek the truth among others which it is easy to obtain from the Church; since the apostles, like a rich man [depositing his money] in a bank, lodged in her hands most copiously all things pertaining to the truth: so that every man, whosoever will, can draw from her the water of life. For she is the entrance to life; all others are thieves and robbers. On this account are we bound to avoid them, but to make choice of the thing pertaining to the Church with the utmost diligence, and to lay hold of the tradition of the truth. For how stands the case? Suppose there arise a dispute relative to some important question among us, should we not have recourse to the most ancient Churches with which the apostles held constant intercourse, and learn from them what is certain and clear in regard to the present question? For how should it be if the apostles themselves had not left us writings? Would it not be necessary, [in that case,] to follow the course of the tradition which they handed down to those to whom they did commit the Churches?
2. To which course many nations of those barbarians who believe in Christ do assent, having salvation written in their hearts by the Spirit, without paper or ink, and, carefully preserving the ancient tradition, believing in one God, the Creator of heaven and earth, and all things therein, by means of Christ Jesus, the Son of God; who, because of His surpassing love towards His creation, condescended to be born of the virgin, He Himself uniting man through Himself to God, and having suffered under Pontius Pilate, and rising again, and having been received up in splendour, shall come in glory, the Saviour of those who are saved, and the Judge of those who are judged, and sending into eternal fire those who transform the truth, and despise His Father and His advent. Those who, in the absence of written documents, have believed this faith, are barbarians, so far as regards our language; but as regards doctrine, manner, and tenor of life, they are, because of faith, very wise indeed; and they do please God, ordering their conversation in all righteousness, chastity, and wisdom. If any one were to preach to these men the inventions of the heretics, speaking to them in their own language, they would at once stop their ears, and flee as far off as possible, not enduring even to listen to the blasphemous address. Thus, by means of that ancient tradition of the apostles, they do not suffer their mind to conceive anything of the [doctrines suggested by the] portentous language of these teachers, among whom neither Church nor doctrine has ever been established.
3. For, prior to Valentinus, those who follow Valentinus had no existence; nor did those from Marcion exist before Marcion; nor, in short, had any of those malignant-minded people, whom I have above enumerated, any being previous to the initiators and inventors of their perversity. For Valentinus came to Rome in the time of Hyginus, flourished under Pius, and remained until Anicetus. Cerdon, too, Marcion's predecessor, himself arrived in the time of Hyginus, who was the ninth bishop. Coming frequently into the Church, and making public confession, he thus remained, one time teaching in secret, and then again making public confession; but at last, having been denounced for corrupt teaching, he was excommunicated from the assembly of the brethren. Marcion, then, succeeding him, flourished under Anicetus, who held the tenth place of the episcopate. But the rest, who are called Gnostics, take rise from Menander, Simon's disciple, as I have shown; and each one of them appeared to be both the father and the high priest of that doctrine into which he has been initiated. But all these (the Marcosians) broke out into their apostasy much later, even during the intermediate period of the Church.
Source: Adversus haereses (New Advent)