The nature of God is paramount in apologetics, so today we will continue to explore what that nature entails. The article below explains God’s Knowledge:
Knowledge. God Knows Himself. If God is absolutely simple, can he know himself? All knowledge involves both a knower and a known. But God has no such duality. Aquinas argues that in self-knowledge the knower and known are identical. Hence, God can only know himself through himself (ibid., 1a.14.2). Since God is simple, he knows himself simply.
God also knows himself perfectly. Something is known perfectly when its potential to be known is completely realized. And there is no unactualized potentiality to know himself. Therefore, God’s self-knowledge is completely actualized (ibid., 1a.14.3).
God’s knowledge is identical with his essence. For if God’s acts of knowledge were really distinct from his essence, then they would be related as actuality to potentiality. But there can be no potentiality in God. Therefore, God’s knowledge and essence are really identical (ibid., 1a.14.4). This does not mean that God cannot know things other than himself. For God is the efficient cause of all things.
God Knows and Does. Even though God knows other things than himself, nonetheless, he knows them through himself. For God does not know other things through himself either successively or inferentially but simultaneously and intuitively (ibid., 1a.14.7, ad 2). God’s knowledge is more perfect because he does not have to know things discursively through their causes but knows them directly and intuitively (ibid., 1a.14.7 ad 3, 4). God not only knows all things in and through himself, but he also causes all things by his knowledge. God causes all things by his being, but God’s being and his knowledge are identical (ibid., 1a.14.8). This does not mean that creation is eternal because he is eternal. For God causes all things as they are in his knowledge. But that creation should be eternal was not in God’s knowledge (ibid., 1a.14.8, ad 2).
An effect pre-exists in the mind of its efficient cause. Hence, whatever exists must pre-exist in God, who is its efficient cause. God knows all of the various kinds of perfection in himself, as well as those which can participate in his likeness. Therefore, God knows whatever exists perfectly, insofar as it pre-exists in him (ibid., 1a.14.5).
God Knows Every Creature Ideally. God knows his own essence perfectly. And knowing his essence perfectly entails knowing it according to every mode by which it can be known, namely, in itself and as participated in by creatures. But every creature has its own proper form, in which it is like God. It follows, therefore, that God knows the form or idea of every creature as it is modeled after him. Perfect knowledge involves the ability to distinguish one thing from another. That is, he knows not only what things have in common (esse) but how they differ (essence). Therefore, God knows all things in their individual essences. But all things pre-exist in God’s knowledge. Therefore, all things pre-exist in God’s knowledge, not only with regard to their existence but also with regard to their individual essences.
The basis for what God knows is his own essence, but the extent of what he knows is not limited to that one essence but reaches to all things like it (ibid., 1a.15.2). God’s knowledge of all things in himself does not mean that he only knows other things in general but not in particular. For God’s knowledge extends as far as does his causality. And God’s causality extends to singular things, since he is the cause of every individual thing. Therefore, God knows singular things (ibid., 1a.14.11). God has a perfect knowledge of everything. And to know something only in general but not in particular is improper knowledge. So, God knows everything properly. That is, he does not know the radii of circles merely by knowing the center; he knows the radii as well as the center.
God Knows Evil. For perfect knowledge of things must include knowing all that can occur to them. Evil can occur as a corruption of good things. Hence, God can know evil (see Evil, Problem of). But things are knowable in the way in which they exist. Evil is a privation in good things. Therefore, God knows evil as a privation in a good (ibid., 1a.14.10).
God Knows Changing Things. Since God is unchanging and his knowledge is identical with his essence, he knows past, present, and future in one eternal now. Therefore, when time changes, God’s knowledge does not change, since he knew it in advance. God knows change, but not in the way we know it, in successive time frames. From eternity God knows the whole of before and after the temporal now of human history (ibid., 1a.14.15).
God knows the same things we do, but he does not know them the same way we know them. Our knowledge is discursive, moving from premises to conclusions. In human knowledge there is twofold discursiveness: One thing is known after another, and one thing is known through another. But God cannot know things sequentially, since he is timeless and knows all things eternally at once. Nor can God know things inferentially, for he is simple and knows all things through the oneness of himself. Therefore, God cannot know anything discursively (sequentially, from topic to topic), inasmuch as discursive knowledge implies a limitation to consider one thing at a time on the part of the knower (ibid., 1a.14.7).
God Knows All Possibilities. By knowing himself perfectly God knows perfectly all the different ways his perfections can be shared by others. For there is within the essence of God all the knowledge of all possible kinds of things his will could actualize. Hence, God knows all the particular things that could ever be actualized (ibid., 1a.14.6).
God’s Knowledge Allows Free Will. Pulling these strands of thought about God’s knowledge together shows us how God’s sovereignty works alongside human free will. God’s knowledge is not simply of the actual; he also knows all possible sorts of potential. He knows what is and ever could-be. For God knows whatever is in any way it can be known. Now both the actual and the potential are real. Only the impossible has no reality. Thus, whatever is potential is real. It follows that God can know what is potential as well as what is actual (ibid., 1a.14.9).
This means that God can know future contingents, that is, things that are dependent on free choice. For the future is a potential that pre-exists in God. And God knows whatever exists in himself as the cause of those things (ibid., 1a.14.13). Since God is a timeless being, he knows all of time in one eternal now. But the future is part of time. Therefore, God knows the future, including the free acts to be performed in it. Of course, whatever God knows is known infallibly, since God cannot err in his knowledge. Future contingents are known infallibly. They are contingent with regard to their immediate cause (human free choice) but necessary with regard to God’s knowledge. God can do this without eliminating free choice, for an omniscient being can know whatever is not impossible to know. And it is not impossible for a timeless being to know a necessary end caused by a contingent means. God can know a must-be through a may-be but not a can’t-be.
Therefore, an omniscient Being knows future actions as necessarily true events. If an action will occur and God knows it, then that event must occur, for an omniscient Mind cannot be wrong about what it knows. Therefore the statement “Everything known by God must necessarily be” is true if it refers to the statement of the truth of God’s knowledge, but it is false if it refers to the necessity of the contingent events (ibid., 1a.14.5).
Geisler, N. L. (1999). In Baker encyclopedia of Christian apologetics (pp. 285–286). Grand Rapids, MI: Baker Books.