4 Again, hear concerning the title Son of God, by which we have called Him. They say that “though God has no son, you make that crucified Jesus, the firstborn son of God.” Yet He called Israel “My first-born,” when He sent to Pharaoh through Moses and said to him, Israel is My first-born; I have said unto you, let My Son go to serve Me, and if you are not willing to let (him) go, lo! I will slay your son, your firstborn. And also through the Prophet He testified concerning this, and reproved them and said to the people, Out of Egypt have I called My son.
As I called them, so they went and worshipped Baal and offered incense to the graven images. And Isaiah said concerning them, Children have I reared and brought up, and they have rebelled against Me. And again it is written, You are the children of the Lord your God. And about Solomon He said, He shall be to Me a son, and I will be to him a Father. So also we call the Christ, the Son of God, for through Him we have gained the knowledge of God; even as He called Israel My firstborn son, and as He said concerning Solomon, He shall be to Me a son.
And we call Him God, even as He surnamed Moses by His own Name. And also David said concerning them:— You are Gods and children of the Highest, all of you. And when they amended not themselves, therefore He said concerning them:— As men shall you die, and as one of the princes shall you fall.
Source: Demonstrations (New Advent)