40 Thus God is in God, and it is God in Whom God dwells. But how is There is no God beside You true, if God be within Him? Heretic! In support of your confession of a solitary Father you employ the words, There is no God beside Me; what sense can you assign to the solemn declaration of God the Father, There is no God beside You, if your explanation of There is no God beside Me be a denial of the Godhead of the Son? To whom, in that case, can God have said, There is no God beside You? You cannot suggest that this solitary Being said it to Himself.
It was to the King Whom He summoned that the Lord said, by the mouth of the men of stature who worshipped and made supplication, For God is in You. The facts are inconsistent with solitude. In You implies that there was One present within range, if I may say so, of the Speaker's voice. The complete sentence, God is in You, reveals not only God present, but also God abiding in Him Who is present. The words distinguish the Indweller from Him in Whom He dwells, but it is a distinction of Person only, not of character.
God is in Him, and He, in Whom God is, is God. The residence of God cannot be within a nature strange and alien to His own. He abides in One Who is His own, born from Himself. God is in God, because God is from God. For You are God, and we knew it not, O God of Israel, the Saviour.
Source: On the Trinity (New Advent)