The virgin birth was unexpected

For what great thing or what sign should have been in this, that a young woman conceiving by a man should bring forth,—a thing which happens to all women that produce offspring? But since an unlooked-for salvation was to be provided for men through the help of God, so also was the unlooked-for birth from a virgin accomplished; God giving this sign, but man not working it out. On this account

Irenaeus of Lyons
Against Heresies, Book III
Chapter XXI

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

Christ is beyond all men who ever lived, God and Lord

He who “was not born either by the will of the flesh, or by the will of man,” is the Son of man, this is Christ, the Son of the living God. For I have shown from the Scriptures, that no one of the sons of Adam is as to everything, and absolutely, called God, or named Lord. But that He is Himself in His own right, beyond all men who ever lived, God, and Lord, and King Eternal, and the Incarnate Word, proclaimed by all the prophets, the apostles, and by the Spirit Himself, may be seen by all who have attained to even a small portion of the truth. Now,

Irenaeus of Lyons
Against Heresies, Book III
Chapter XIX

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