Irenaeus of Lyons, Against Heresies 3.4

Therefore, since these demonstrations stand, there is no need to go searching for the truth among others, when it is so easy to receive it from the Church. The Apostles, like men entrusting a treasure to a rich repository, deposited the fullness of truth in her, so that anyone who wishes may draw from her the drink of life. This is the entrance to life; all the rest, however, are thieves and robbers. For this reason we must avoid them, but we must love the things that belong to the Church with the greatest diligence and lay hold of the tradition of truth.

What if a dispute arose over some minor question? Would it not be necessary to turn to the most ancient churches, where the Apostles conversed, and learn from them what is certain and clear regarding the present issue? What if the Apostles had not even left us the Scriptures? Would it not then be necessary to follow the order of tradition, which they handed down to those to whom they entrusted the churches?

Many nations of barbarians who believe in Christ assent to this order. They possess salvation written in their hearts by the Spirit, without paper or ink, carefully preserving the ancient tradition. They believe in one God, the maker of heaven and earth and all that is in them, through Christ Jesus, the Son of God. Because of his surpassing love for his creation, he endured his birth from the Virgin, himself uniting man to God. He suffered under Pontius Pilate, rose again, was received in glory, and will come in glory as the Savior of those who are saved and the Judge of those who are judged. He will send into eternal fire the transgressors of truth and the despisers of his Father and his coming.

Those who have believed this faith without written documents are, in respect to our speech, barbarians; but in respect to judgment, custom, and conduct, they are supremely wise because of their faith, and they please God, living in all justice, chastity, and wisdom. If anyone announces to them the inventions of the heretics, speaking in the heretics' own tongue, they immediately stop their ears and flee far away, refusing even to endure such blasphemous talk. Thus, through that ancient tradition of the elders, they do not admit the heretics' portentous speech even into the conception of their minds, for there was no assembly among them and no instituted doctrine.

Before Valentinus came, those who are from Valentinus did not exist; nor did those who are from Marcion exist before Marcion; nor did any of the other malignant opinions enumerated above exist to become the authors of their perversity. Valentinus came to Rome under Hyginus, flourished under Pius, and remained until Anicetus. Cerdon, Marcion's predecessor, also came to Rome under Hyginus, the eighth bishop, frequently entering the Church and making confession. He ended this way: sometimes teaching secretly, sometimes making confession, sometimes corrected by others for his bad teaching, and he kept away from the gathering of the devout. Marcion succeeded him and grew strong under Anicetus, who held the tenth place in the episcopate. The rest who are called Gnostics received their start from Menander, Simon's disciple, as we have shown; each one, having shared in a particular opinion, became its father and high priest. All these men arose much later, in the midst of the Church's times, to launch their apostasy.