41 My next book is devoted to the refutation of your denial that God is in God; for the prophet continues, All that resist Him shall be ashamed and confounded and shall walk in confusion. This is God's sentence, passed upon your unbelief. You set yourself in opposition to Christ, and it is on His account that the Father's voice is raised in solemn reproof; for He, Whose Godhead you deny, is God. And you deny it under cloak of reverence for God, because He says, There is no other God beside Me.
Submit to shame and confusion; the Unoriginate God has no need of the dignity you offer; He has never asked for this majesty of isolation which you attribute to Him. He repudiates your officious interpretation which would twist His words, There is no other God beside Me, into a denial of the Godhead of the Son Whom He begot from Himself. To frustrate your purpose of demolishing the Divinity of the Son by assigning the Godhead in some special sense to Himself, He rounds off the glories of the Only-begotten by the attribution of absolute Divinity:— And there is no God beside You.
Why make distinctions between exact equivalents? Why separate what is perfectly matched? It is the peculiar characteristic of the Son of God that there is no God beside Him; the peculiar characteristic of God the Father that there is no God apart from Him. Use His words concerning Himself; confess Him in His own terms, and entreat Him as King; For God is in You, and there is no God beside You. For You are God, and we knew it not, O God of Israel, the Saviour. A confession couched in words so reverent is free from the taint of presumption: its terms can excite no repugnance.
Above all, we must remember that to refuse it means shame and ignominy. Brood in thought over these words of God; employ them in your confession of Him, and so escape the threatened shame. For if you deny the Divinity of the Son of God, you will not be augmenting the glory of God by adoring Him in lonely majesty; you will be slighting the Father by refusing to reverence the Son. In faith and veneration confess of the Unoriginate God that there is no God beside Him; claim for God the Only-begotten that apart from Him there is no God.
Source: On the Trinity (New Advent)