In Against Heresies Book II chapter 19, Irenaeus( c. 120- 28th of June 202 A.D), a bishop of Lyons, taught by a claimed disciple of apostle John, Polycarp ( c.69 – 155 A.D) the bishop of Smyrna, we find early church’s understanding of Isaiah 9:6, to which I wish to share:

For this reason [it is said], “Who shall declare His generation?” since “He is a man, and who shall recognise Him?” But he to whom the Father which is in heaven has revealed Him, knows Him, so that he understands that 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, the Scriptures would not have testified these things of Him, if, like others, He had been a mere man. But that He had, beyond all others, in Himself that pre-eminent birth which is from the Most High Father, and also experienced that pre-eminent generation which is from the Virgin, the divine Scriptures do in both respects testify of Him: also, that He was a man without comeliness, and liable to suffering; that He sat upon the foal of an ass; that He received for drink, vinegar and gall; that He was despised among the people, and humbled Himself even to death; and that He is the holy Lord, the Wonderful, the Counsellor, the Beautiful in appearance, and the Mighty God, coming on the clouds as the Judge of all men;—all these things did the Scriptures prophesy of Him.

Source:

The Ante-Nicene Fathers, Volume I: The Apostolic Fathers with Justin Martyr and Irenaeus. 1885 (A. Roberts, J. Donaldson & A. C. Coxe, Ed.) (449). Buffalo, NY: Christian Literature Company. (paragraphs added)