He is God and man, as prophesied in Isaiah

Carefully, then, has the Holy Ghost pointed out, by what has been said, His birth from a virgin, and His essence, that He is God (for the name Emmanuel indicates this). And He shows that He is a man, when He says, “Butter and honey shall He eat;” and in that He terms Him a child also, [in saying,] “before He knows good and evil;” for these are all the tokens of a human infant. But that He “will not consent to evil, that He may choose that which is good,”—this is proper to God; that by the fact, that He shall eat butter and honey, we should not understand that He is a mere man only, nor, on the other hand, from the name Emmanuel, should suspect Him to be God without flesh.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XXI

The Son of God, who is God, came from Judah

And that it is from that region which is towards the south of the inheritance of Judah that the Son of God shall come, who is God, and who was from Bethlehem, where the Lord was born [and] will send out His praise through all the earth…

…Thus he indicates in clear terms that He is God, and that His advent was [to take place] in Bethlehem, and from Mount Effrem which is towards the south of the inheritance, and that [He is] man.

Irenaeus of Lyons
Against Heresies, Book III
Chapter XX

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

Jesus sanctified humanity having lived the full human experience

For He came to save all through means of Himself—all, I say, who through Him are born again to God—infants, and children, and boys, and youths, and old men. He therefore passed through every age, becoming an infant for infants, thus sanctifying infants; a child for children, thus sanctifying those who are of this age, being at the same time made to them an example of piety, righteousness, and submission; a youth for youths, becoming an example to youths, and thus sanctifying them for the Lord. So likewise He was an old man for old men, that He might be a perfect Master for all, not merely as respects the setting forth of the truth, but also as regards age, sanctifying at the same time the aged also, and becoming an example to them likewise.

Irenaeus of Lyons
Against Heresies, Book II
Chapter XXII

God was born of a Virgin

If, then, we know that God revealed Himself in so many forms to Abraham, and to Jacob, and to Moses, how are we at a loss, and do not believe that, according to the will of the Father of all things, it was possible for Him to be born man of the Virgin, especially after we have such Scriptures, from which it can be plainly perceived that He became so according to the will of the Father?

Justin Martyr
Dialogue with Trypho
Chapter LXXV