Objection 1. It seems that in God there is not knowledge. For knowledge is a habit; and habit does not belong to God, since it is the mean between potentiality and act. Therefore knowledge is not in God.
Objection 2. Further, since science is about conclusions, it is a kind of knowledge caused by something else which is the knowledge of principles. But nothing is caused in God; therefore science is not in God.
Objection 3. Further, all knowledge is universal, or particular. But in God there is no universal or particular (3, 5). Therefore in God there is not knowledge.
On the contrary, The Apostle says, "O the depth of the riches of the wisdom and of the knowledge of God" (Romans 11:33).
I answer that, In God there exists the most perfect knowledge. To prove this, we must note that intelligent beings are distinguished from non-intelligent beings in that the latter possess only their own form; whereas the intelligent being is naturally adapted to have also the form of some other thing; for the idea of the thing known is in the knower. Hence it is manifest that the nature of a non-intelligent being is more contracted and limited; whereas the nature of intelligent beings has a greater amplitude and extension; therefore the Philosopher says (De Anima iii) that "the soul is in a sense all things." Now the contraction of the form comes from the matter. Hence, as we have said above (Question 7, Article 1) forms according as they are the more immaterial, approach more nearly to a kind of infinity. Therefore it is clear that the immateriality of a thing is the reason why it is cognitive; and according to the mode of immateriality is the mode of knowledge. Hence it is said in De Anima ii that plants do not know, because they are wholly material. But sense is cognitive because it can receive images free from matter, and the intellect is still further cognitive, because it is more separated from matter and unmixed, as said in De Anima iii. Since therefore God is in the highest degree of immateriality as stated above (Question 7, Article 1), it follows that He occupies the highest place in knowledge.
Reply to Objection 1. Because perfections flowing from God to creatures exist in a higher state in God Himself (4, 2), whenever a name taken from any created perfection is attributed to God, it must be separated in its signification from anything that belongs to that imperfect mode proper to creatures. Hence knowledge is not a quality of God, nor a habit; but substance and pure act.
Reply to Objection 2. Whatever is divided and multiplied in creatures exists in God simply and unitedly (13, 4). Now man has different kinds of knowledge, according to the different objects of His knowledge. He has "intelligence" as regards the knowledge of principles; he has "science" as regards knowledge of conclusions; he has "wisdom," according as he knows the highest cause; he has "counsel" or "prudence," according as he knows what is to be done. But God knows all these by one simple act of knowledge, as will be shown (7). Hence the simple knowledge of God can be named by all these names; in such a way, however, that there must be removed from each of them, so far as they enter into divine predication, everything that savors of imperfection; and everything that expresses perfection is to be retained in them. Hence it is said, "With Him is wisdom and strength, He hath counsel and understanding" (Job 12:13).
Reply to Objection 3. Knowledge is according to the mode of the one who knows; for the thing known is in the knower according to the mode of the knower. Now since the mode of the divine essence is higher than that of creatures, divine knowledge does not exist in God after the mode of created knowledge, so as to be universal or particular, or habitual, or potential, or existing according to any such mode.
Source: Summa Theologica (New Advent)