9 Furthermore, we must prove that this Jesus was beforehand promised from ancient times in the Prophets, and was called the Son of God. David said:— You are My Son; today have I begotten You. Again he said:— In the glories of holiness, from the womb, from of old, have I begotten you, a child. And Isaiah said:— Unto us a child is born, unto us a Son is given, and His government was upon His shoulder, and His Name shall be called Wonderful, and Counsellor, and mighty God of the ages, and Prince of peace.
And to the increase of His government and to His peace there is no end. Therefore tell me, O wise doctor of Israel, who is He that was born and whose name was called Child and Son and Wonderful and Counsellor, the mighty God of the ages, and Prince of peace, to the increase of whose government and to whose peace (he said), there is no end? For if we call Christ the Son of God, David taught us (this); and that we call Him God, this we learned from Isaiah. And His government was laid upon His shoulder; for He bare His cross, and went out from Jerusalem.
And that He was born as a child, Isaiah again said:— Lo, the virgin shall conceive and bear; and His name shall be called Immanuel, which is, our God with us.
Source: Demonstrations (New Advent)