(CG 1/044). That God has Understanding (Book 1: Of God As He Is In Himself) (Summa Contra Gentiles) (Aquinas, Thomas, St.)

TO understand is an act of an intelligent being, existing in that being, not passing out to anything external, as the act of warming passes out to the object warmed: for an intelligible object suffers nothing from being understood, but the intelligence that understands it is perfected thereby. But whatever is in God is the divine essence. Therefore the act of understanding in God is the divine essence.

5. Every substance is for the sake of its activity. If therefore the activity of God is anything else than the divine substance, His end will be something different from Himself; and thus God will not be His own goodness, seeing that the good of every being is its end.

From the act of understanding in God being identical with His being, it follows necessarily that the act of His understanding is absolutely eternal and invariable, exists in actuality only, and has all the other attributes that have been proved of the divine being. God then is not potentially intelligent, nor does He begin anew to understand anything, nor does He undergo any change or composition in the process of understanding.