Scripture is used to understand scripture

But while I bring out by these proofs the truths of Scripture, and set forth briefly and compendiously things which are stated in various ways, do thou also attend to them with patience, and not deem them prolix; taking this into account, that proofs [of the things which are] contained in the Scriptures cannot be shown except from the Scriptures themselves.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XII

Four covenants, four gospels

For this reason were four principal (καθολικαί) covenants given to the human race: one, prior to the deluge, under Adam; the second, that after the deluge, under Noah; the third, the giving of the law, under Moses; the fourth, that which renovates man, and sums up all things in itself by means of the Gospel, raising and bearing men upon its wings into the heavenly kingdom.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XI

God (the Son) shall appear openly

He [here] refers to the Father and the Son, and those who have received the adoption; but these are the Church. For she is the synagogue of God, which God—that is, the Son Himself—has gathered by Himself. Of whom He again speaks: “The God of gods, the Lord hath spoken, and hath called the earth.” Who is meant by God? He of whom He has said, “God shall come openly, our God, and shall not keep silence;” that is, the Son, who came manifested to men who said, “I have openly appeared to those who seek Me not.”

Irenaeus of Lyons
Against Heresies, Book III
Chapter VI

Barbarians believe in Christ in the absence of scripture

…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

…Those who, in the absence of written documents, have believed this faith, are barbarians,

…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.

Irenaeus of Lyons
Against Heresies, Book III
Chapter IV

Doctrinal disputes are resolved by the ancient Churches founded by the Apostles

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?

Irenaeus of Lyons
Against Heresies, Book III
Chapter IV

 

Polycarp came to Rome and defended against the Marcionites

He it was who, coming to Rome in the time of Anicetus caused many to turn away from the aforesaid heretics to the Church of God, proclaiming that he had received this one and sole truth from the apostles,—that, namely, which is handed down by the Church.

Irenaeus of Lyons
Against Heresies, Book III
Chapter III

The ecclesiastical tradition of the apostles is handed down

In this order, and by this succession, the ecclesiastical tradition from the apostles, and the preaching of the truth, have come down to us. And this is most abundant proof that there is one and the same vivifying faith, which has been preserved in the Church from the apostles until now, and handed down in truth.

Irenaeus of Lyons
Against Heresies, Book III
Chapter III

The Roman tradition was established by Peter and Paul and succeeded by bishops

…[we do this, I say,] by indicating that tradition derived from the apostles, of the very great, the very ancient, and universally known Church founded and organized at Rome by the two most glorious apostles, Peter and Paul; as also [by pointing out] the faith preached to men, which comes down to our time by means of the successions of the bishops. For it is a matter of necessity that every Church should agree with this Church, on account of its pre-eminent authority, that is, the faithful everywhere, inasmuch as the apostolical tradition has been preserved continuously by those [faithful men] who exist everywhere.

Irenaeus of Lyons
Against Heresies, Book III
Chapter III