In this post I will be citing from another work of St. Irenaeus where he clearly affirms the perfect eternal Deity and Incarnation of the Lord Jesus Christ. The name of this work is Demonstration of the Apostolic Preaching. All bold emphasis will be mine.
43. So then we must believe God in all things, for in all things God is true. Now that there was a Son of God, and that He existed not only before He appeared in the world, but also before the world was made, Moses, who was the first that prophesied164 says in Hebrew: Baresith bara Elowin basan benuam samenthares. And this, translated into our language,165 is: “The Son in the beginning: God established then the heaven and the earth.”166 This Jeremiah the prophet also testified, saying thus: Before the morning-star I begat thee: and before the sunlight; (is) thy name;167 and that is, before the creation of the world; for together with the world the stars were made. And again the same says: Blessed is he who was, before he became man.168 Because, for God, the Son was (as) the beginning before the creation of the world;169 but for us (He was) then, when He appeared; and before that He was not for us, who knew Him not.170 Wherefore also His disciple John, in teaching us who is the Son of God, who was with the Father before the world was made, and that all the things that were made were made by Him, says thus: In beginning was the Word, and the Word was with God, and the Word was God. The same was in the beginning with God. All things were made by Him, and without Him was not anything made: showing with certainty that the Word, who was in the beginning with the Father, and by whom all things were made, this is His Son.
44. And again Moses tells how the Son of God drew near to hold converse with Abraham: And God appeared unto him by the oak of Mamre in the middle of the day. And looking up with his eyes he beheld, and, lo, three men stood over against him. And he bowed himself down to the earth, and said: Lord, if indeed I have found favour in thy sight. And all that which follows he spake with the Lord, and the Lord spake with him. Now two of the three were angels; but one was the Son of God, with whom also Abraham spake, pleading on behalf of the men of Sodom, that they should not perish if at least ten righteous should be found there. And, whilst these were speaking, the two angels entered into Sodom, and Lot received them. And then the Scripture says: And the Lord rained upon Sodom and Gomorrah brimstone and fire from the Lord out of heaven: that is to say, the Son, who spake with Abraham, being Lord, received power to punish the men of Sodom from the Lord out of heaven, even from the Father who rules over all.171 So Abraham was a prophet and saw things to come, which were to take place in human form even the Son of God, that He should speak with men and eat with them, and then should bring in the judgment from the Father, having received from Him who rules over all the power to punish the men of Sodom.
45. And Jacob, when he went into Mesopotamia, saw Him in a dream, standing upon the ladder, that is the tree which was set up from earth to heaven;172 for thereby they that believe on Him go up to the heavens. For His sufferings are our ascension on high. And all such visions point to the Son of God, speaking with men and being in their midst. For it was not the Father of all, who is not seen by the world, the Maker of all who has said: Heaven is my throne, and earth is my footstool: what house will ye build me, or what is the place of my rest? (Acts vii. 49) and who comprehendeth the earth with his hand, and with his span the heaven173—it was not He that came and stood in a very small space and spake with Abraham; but the Word of God, who was ever with mankind, and made known beforehand what should come to pass in the future, and taught men the things of God.
46. He it is who spake with Moses in the bush, and said: Seeing have I seen the affliction of my people that is in Egypt; and I am come down to deliver them. He it is who came forth and came down for the deliverance of the oppressed, bringing us out from the power of the Egyptians, that is, from all idolatry and impiety; and delivering us from the Red Sea, that is, delivering us from the deadly confusion of the Gentiles and the grievous vexation of their blasphemy. For in them the Word of God prepared and rehearsed beforehand the things concerning us. Then He set forth in types beforehand that which was to be; how in very truth He has brought us out from the cruel service of the Gentiles, and a stream of water in the desert has He made to flow forth in abundance from a rock; and that rock is Himself; and has given twelve fountains, that is, the teaching of the twelve apostles. And the obstinate unbelievers He brought to an end and consumed in the wilderness; but those who believed on Him, and in malice were children,174 He made to enter into the inheritance of the fathers; whom not Moses, but Jesus puts in possession of the heritage:175 who also delivers us from Amalek by the expansion of His hands,176 and brings us to the kingdom of the Father.
47. So then the Father is Lord and the Son is Lord,177 and the Father is God and the Son is God; for that which is begotten of God is God.178 And so in the substance and power of His being there is shown forth one God; but there is also according to the economy of our redemption both Son and Father. Because to created things the Father of all is invisible and unapproachable,179 therefore those who are to draw near to God must have their access to the Father through the Son. And yet more plainly and evidently does David speak concerning the Father and the Son as follows: Thy throne, O God is for ever and ever: thou hast loved righteousness and hated unrighteousness:180 therefore God hath anointed thee with the oil of gladness above thy fellows. (Ps. xlv. 6 f.) For the Son, as being God, receives from the Father, that is, from God,181 the throne of the everlasting kingdom, and the oil of anointing above His fellows. The oil of anointing is the Spirit,182 wherewith He has been anointed; and His fellows are prophets and righteous men and apostles, and all who receive the fellowship of His kingdom, that is to say, His disciples.
48. And again David says: The Lord said unto my Lord: Sit on my right hand, until I make thy enemies thy footstool. The rod of thy strength shall the Lord send forth from Sion; and rule thou in the midst of thy enemies. With thee in the beginning183 in the day of thy power, in the brightness of the holy ones: from the womb before the morning-star I begat thee. The Lord sware and will not repent: Thou art a priest for ever after the order of Melchisedec. And the Lord on thy right hand hath broken in pieces kings in the day of wrath: he shall judge among the Gentiles, he shall fill up the ruins, and shall break in pieces the heads of many on the earth.184 He shall drink of the brook in the way: therefore shall he lift up the head. Now hereby he proclaimed that He came into being before all, and that He rules over the Gentiles and judges all mankind and the kings who now hate Him and persecute His name; for these are His enemies: and in calling Him God’s priest for ever, he declared His immortality. And therefore he said: He shall drink of the brook in the way; therefore shall he lift up the head; proclaiming the exaltation with glory that followed on His humanity and humiliation and in gloriousness.185
49. And again Isaiah the prophet says: Thus saith the Lord God to my Anointed the Lord,186 whose right hand I have held, that the Gentiles should hearken before him. And how the Christ is called Son of God and King of the Gentiles, that is, of all mankind; and that He not only is called but is Son of God and King of all, David declares thus: The Lord said unto me: Thou art my Son, this day have I begotten thee. Ask of me and I will give thee the Gentiles for thy inheritance, and for a possession the utmost parts of the earth. These things were not said of David; for neither over the Gentiles nor over the utmost parts did he rule, but only over the Jews. So then it is plain that the promise to the Anointed to reign over the utmost parts of the earth is to the Son of God, whom David himself acknowledges as his Lord, saying thus: The Lord said unto my Lord, Sit on my right hand, and so forth, as we have said above. For he means that the Father speaks with the Son; as we showed a little before as to Isaiah, that he said thus: God saith to my Anointed the Lord, that the Gentiles should hearken before him. For the promise is the same by the two prophets, that He should be King: so that the speech of God is addressed to one and the same, I mean, to Christ the Son of God. Forasmuch as David says: The Lord said unto me, it is necessary to say that it is not David who speaks, nor any one of the prophets, in his own person: for it is not a man who speaks the prophecies; but the Spirit of God, assimilating and likening Himself to the persons represented, speaks in the prophets, and utters the words sometimes from Christ and sometimes from the Father.187
50. So then right fitly Christ says through David that He converses with the Father; and right worthily does He say the other things concerning Himself through the prophets; as in other instances, so also after this manner by Isaiah: And now thus saith the Lord, who formed me as his servant from the womb, to gather Jacob and to gather Israel unto him: and I shall be glorified before the Lord, and my God shall be a strength unto me. And he said: A great thing shall it be to thee to be called my servant, to stablish and confirm the tribe of Jacob, and to turn again the dispersion of Israel: and I have set thee for a light of the Gentiles,188 that thou shouldst be for salvation unto the end of the earth.
51. Here, first of all, is seen that the Son of God pre-existed, from the fact that the Father spake with Him,189 and before He was born revealed Him to men: and, next, that He must needs be born a man among men; and that the same God forms Him from the womb, that is, that of the Spirit of God He should be born; and that He is Lord of all men, and Saviour of them that believe on Him, both Jews and others. For the people of the Jews is called Israel in the Hebrew language, from Jacob their father, who was the first to be called Israel: and Gentiles He calls the whole of mankind. And that the Son of the Father calls Himself servant, (this is) on account of His subjection to the Father: for among men also every son is servant of his father.
52. That Christ, then, being Son of God before all the world, is with the Father; and being with the Father190 is also nigh and close and joined unto mankind; and is King of all, because the Father has subjected all things unto Him; and Saviour of them that believe on Him—such things do the Scriptures declare. For it is not feasible and possible to enumerate every scripture in order; and from these you may understand the others also which have been spoken in like manner, believing in Christ, and seeking understanding and comprehension from God, so as to understand what has been spoken by the prophets.
53. And that this Christ, who was with the Father, being the Word of the Father, was thereafter to be made flesh and become man and undergo the process of birth and be born of a virgin and dwell among men, the Father of all bringing about His incarnation191—Isaiah says thus: Therefore the Lord himself shall give you a sign: behold, the virgin shall conceive and shall bring forth a son, and ye shall call him Emmanuel: butter and honey shall he eat; before he knoweth or selecteth the evil, he chooseth the good: for, before the child knoweth good or evil, he rejecteth wickedness to choose the good. So he proclaimed His birth from a virgin; and that He was truly man he declared beforehand by His eating; and also because he called Him the child; and further by giving Him a name; for this is the custom also for one that is born.192 And His name is two-fold: in the Hebrew tongue Messiah Jesus, and in ours Christ Saviour. And the two names are names of works actually wrought. For He was named Christ, because through Him the Father anointed and adorned all things; and because on His coming as man He was anointed with the Spirit of God and His Father. As also by Isaiah He says of Himself: The Spirit of the Lord is upon me: wherefore he hath anointed me to preach good tidings to the poor. And (He was named) Saviour for this, that He became the cause of salvation to those who at that time were delivered by Him from all sicknesses and from death,193 and to those who afterwards believed on Him the author of salvation in the future and for evermore.
54. For this cause then is He Saviour. Now Emmanuel is, being interpreted, With you God;194 or as a yearning cry195 uttered by the prophet, such as this: With us shall be God; according to which it is the explanation and manifestation of the good tidings proclaimed. For Behold, He saith, the virgin shall conceive and shall bring forth a son; and He, being God, is to be with us. And, as if altogether astonished196 at these things, he proclaims in regard to these future events that With us shall be God. And yet again concerning His birth the same. prophet says in another place: Before she that travailed gave birth, and before the pains of travail came on, she escaped and was delivered of a man-child. Thus he showed that His birth from the virgin was unforeseen and unexpected.197 And again the same prophet says: Unto us a son is born, and unto us a child is given:198 and his name shall be called Wonderful Counselor, Mighty God.199
55. He calls Him Wonderful Counselor, meaning of the Father: whereby it is declared that the Father works all things together with Him; as is contained in the first book of Moses which is entitled Genesis: And God said, Let us make man after our image and likeness. For there is seen in this place the Father speaking to the Son,200 the Wonderful Counselor of the Father. Moreover He is also our Counselor, giving advice; not compelling as God,201 even though He is Mighty God, (as) he says; but giving advice that we should forsake ignorance and acquire knowledge, and depart from error and come to the truth, and put away corruption and receive incorruption.
56. And again Isaiah says: And they shall wish that they had been burned with fire: for unto us a child is born, and unto us a son is given; whose government is upon his shoulders, and his name is called Angel of great counsel. For I will bring peace upon the rulers, again peace and health unto him. Great is his rule, and of his peace there is no bound, upon the throne of David and upon his kingdom, to prosper and complete, to aid and undertake, 202 in righteousness and judgment from this time forth and for evermore. For hereby the Son of God is proclaimed both as being born and also as eternal King.203 But they shall wish that they had been burned with fire (is said) of those who believe not on Him, and who have done to Him all that they have done: for they shall say in the judgment, How much better that we had been burned with fire before the Son of God was born, than that, when He was born, we should not have believed on Him. Because for those who died before Christ appeared there is hope that in the judgment of the risen204 they may obtain salvation, even such as feared God and died in righteousness and had in them the Spirit of God, as the patriarchs and prophets and righteous men. But for those who after Christ’s appearing believed not on Him, there is a vengeance without pardon in the judgment. Now in this: Whose government is upon his shoulder, the cross is in a figure declared, on which He was nailed back.205 For that which was and is a reproach to Him, and for His sake to us,206 even the cross, this same is, says he, His government, being a sign of His kingdom. And, Angel of great counsel, he says; that is, of the Father207 whom He hath declared unto us.