(CG 1/049)That God primarily and essentially knows Himself alone (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

WE are said to know a thing when we know its cause. But God Himself by His essence is the cause of being to others. Since therefore He knows His own essence most fully, we must suppose that He knows also other beings.

3. Whoever knows anything perfectly, knows all that can be truly said of that thing, and all its natural attributes. But a natural attribute of God is to be cause of other things. Since then He perfectly knows Himself, He knows that He is a cause: which could not be unless He knew something also of what He has caused, which is something different from Himself, for nothing is its own cause.

Gathering together these two conclusions, it appears that God knows Himself as the primary and essential object of His knowledge, and other things as seen in His essence.