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

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

Jesus, the first-born form the dead, existed before all

Then, at last, He came on to death itself, that He might be “the first-born from the dead, that in all things He might have the pre-eminence,” the Prince of life, existing before all, and going before all.

Irenaeus of Lyons
Against Heresies, Book II
Chapter XXII

Only one God, the Creator

How much safer and more accurate a course is it, then, to confess at once that which is true: that this God, the Creator, who formed the world, is the only God, and that there is no other God besides Him—He Himself receiving from Himself the model and figure of those things which have been made—than that, after wearying ourselves with such an impious and circuitous description, we should be compelled, at some point or another, to fix the mind on some One, and to confess that from Him proceeded the configuration of things created.

Irenaeus of Lyons
Against Heresies, Book II
Chapter XVI

The Church tradition is that God created on His own

For even creation reveals Him who formed it, and the very work made suggests Him who made it, and the world manifests Him who ordered it. The Universal Church, moreover, through the whole world, has received this tradition from the apostles.

Irenaeus of Lyons
Against Heresies Book II
Chapter III

God created purely from conception in his mind

Let them cease, therefore, to affirm that the world was made by any other; for as soon as God formed a conception in His mind, that was also done which He had thus mentally conceived. For it was not possible that one Being should mentally form the conception, and another actually produce the things which had been conceived by Him in His mind.

Irenaeus of Lyons
Against Heresies Book II
Chapter III

Heretics claim angels formed creation, but only God did

Those, moreover, who say that the world was formed by angels, or by any other maker of it, contrary to the will of Him who is the Supreme Father, err first of all in this very point, that they maintain that angels formed such and so mighty a creation, contrary to the will of the Most High God.

…With justice, therefore, according to an analogous process of reasoning, the Father of all will be declared the Former of this world, and not the angels, nor any other [so-called] former of the world, other than He who was its Author, and had formerly been the cause of the preparation for a creation of this kind.

…He formed all things that were made by His Word that never wearies. For this is a peculiarity of the pre-eminence of God, not to stand in need of other instruments for the creation of those things which are summoned into existence. His own Word is both suitable and sufficient for the formation of all things, even as John, the disciple of the Lord, declares regarding Him: “All things were made by Him, and without Him was nothing made.”

Irenaeus of Lyons
Against Heresies Book II
Chapter II