36 But lest these words, For I am, and before Me there is no other God, nor shall be after Me, be made a handle for blasphemous presumption, as proving that the Son is not God, since after the God, Whom no God precedes, there follows no other God, the purpose of the passage must be considered. God is His own best interpreter, but His chosen Servant joins with Him to assure us that there is no God before Him, nor shall be after Him. His own witness concerning Himself is, indeed, sufficient, but He has added the witness of the Servant Whom He has chosen.
Thus we have the united testimony of the Two, that there is no God before Him; we accept the truth, because all things are from Him. We have Their witness also that there shall be no God after Him; but They do not deny that God has been born from Him in the past. Already there was the Servant speaking thus, and bearing witness to the Father; the Servant born in that tribe from which God's elect was to spring. He sets forth also the same truth in the Gospels: Behold, My Servant Whom I have chosen, My Beloved in Whom My soul is well pleased. This is the sense, then, in which God says, There is no other God before Me, nor shall be after Me.
He reveals the infinity of His eternal and unchanging majesty by this assertion that there is no God before or after Himself. But He gives His Servant a share both in the bearing of witness and in the possession of the Name of God.
Source: On the Trinity (New Advent)