The heretics rob humanity of promotion into God

To whom the Word says, mentioning His own gift of grace: “I said, Ye are all the sons of the Highest, and gods; but ye shall die like men.” He speaks undoubtedly these words to those who have not received the gift of adoption, but who despise the incarnation of the pure generation of the Word of God, defraud human nature of promotion into God, and prove themselves ungrateful to the Word of God, who became flesh for them. For it was for this end that the Word of God was made man, and He who was the Son of God became the Son of man, that man, having been taken into the Word, and receiving the adoption, might become the son of God. For by no other means could we have attained to incorruptibility and immortality, unless we had been united to incorruptibility and immortality. But how could we be joined to incorruptibility and immortality, unless, first, incorruptibility and immortality had become that which we also are, so that the corruptible might be swallowed up by incorruptibility, and the mortal by immortality, that we might receive the adoption of sons?

Irenaeus of Lyons
Against Heresies, Book III
Chapter XIX

 

Man becomes one with God by the Incarnation

For He is a most holy and merciful Lord, and loves the human race. Therefore, as I have already said, He caused man (human nature) to cleave to and to become, one with God. For unless man had overcome the enemy of man, the enemy would not have been legitimately vanquished. And again: unless it had been God who had freely given salvation, we could never have possessed it securely. And unless man had been joined to God, he could never have become a partaker of incorruptibility. For it was incumbent upon the Mediator between God and men, by His relationship to both, to bring both to friendship and concord, and present man to God, while He revealed God to man. For, in what way could we be partaken of the adoption of sons, unless we had received from Him through the Son that fellowship which refers to Himself, unless His Word, having been made flesh, had entered into communion with us?

Irenaeus of Lyons
Against Heresies, Book III
Chapter XVIII

The Son of God did not “begin to exist”

For I have shown that the Son of God did not then begin to exist, being with the Father from the beginning; but when He became incarnate, and was made man, He commenced afresh the long line of human beings, and furnished us, in a brief, comprehensive manner, with salvation; so that what we had lost in Adam—namely, to be according to the image and likeness of God—that we might recover in Christ Jesus.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XVIII

The Holy Spirit is “water from heaven” and makes all one in Christ

Wherefore also the Lord promised to send the Comforter,3620 who should join us to God. For as a compacted lump of dough cannot be formed of dry wheat without fluid matter, nor can a loaf possess unity, so, in like manner, neither could we, being many, be made one in Christ Jesus without the water from heaven.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XVII

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