35 You know the words, Hear, O Israel, the Lord your God is One; would that you knew them aright! As you interpret them, I seek in vain for their sense. It is said in the Psalms, God, Your God, has anointed You. Impress upon the reader's mind the distinction between the Anointer and the Anointed; discriminate between the You and the Your: make it clear to Whom and of Whom the words are spoken. For this definite confession is the conclusion of the preceding passage, which runs thus; Your throne, O God, is for ever and ever; the sceptre of Your kingdom is a right sceptre.
You have loved righteousness and hated iniquity. And then he continues, Therefore God, Your God, has anointed You. Thus the God of the eternal kingdom, in reward for His love of righteousness and hatred of iniquity, is anointed by His God. Surely some broad difference is drawn, some gap too wide for our mental span, between these names? No; the distinction of Persons is indicated by You and Your, but nothing suggests a difference of nature. Your points to the Author, You to Him Who is the Author's offspring.
For He is God from God, as these same words of the prophet declare, God, Your God, has anointed You. And His own words bear witness that there is no God anterior to God the Un-originate; Be My witnesses, and I am witness, says the Lord God, and My Servant Whom I have chosen, that you may know and believe and understand that I am, and before Me there is no other God, nor shall be after Me. Thus the majesty of Him that has no beginning is declared, and the glory of Him that is from the Unoriginate is safeguarded; for God, Your God, has anointed You.
That word Your declares His birth, yet does not contradict His nature; Your God means that the Son was born from Him to share the Godhead. But the fact that the Father is God is no obstacle to the Son's being God also, for God, Your God, has anointed You. Mention is made both of Father and of Son; the one title of God conveys the assurance that in character and majesty They are One.
Source: On the Trinity (New Advent)