Objection 1. It seems that the knowledge of God is discursive. For the knowledge of God is not habitual knowledge, but actual knowledge. Now the Philosopher says (Topic. ii): "The habit of knowledge may regard many things at once; but actual understanding regards only one thing at a time." Therefore as God knows many things, Himself and others, as shown above (2,5), it seems that He does not understand all at once, but discourses from one to another.
Objection 2. Further, discursive knowledge is to know the effect through its cause. But God knows things through Himself; as an effect (is known) through its cause. Therefore His knowledge is discursive.
Objection 3. Further, God knows each creature more perfectly than we know it. But we know the effects in their created causes; and thus we go discursively from causes to things caused. Therefore it seems that the same applies to God.
On the contrary, Augustine says (De Trin. xv), "God does not see all things in their particularity or separately, as if He saw alternately here and there; but He sees all things together at once."
I answer that, In the divine knowledge there is no discursion; the proof of which is as follows. In our knowledge there is a twofold discursion: one is according to succession only, as when we have actually understood anything, we turn ourselves to understand something else; while the other mode of discursion is according to causality, as when through principles we arrive at the knowledge of conclusions. The first kind of discursion cannot belong to God. For many things, which we understand in succession if each is considered in itself, we understand simultaneously if we see them in some one thing; if, for instance, we understand the parts in the whole, or see different things in a mirror. Now God sees all things in one (thing), which is Himself. Therefore God sees all things together, and not successively. Likewise the second mode of discursion cannot be applied to God.
First, because this second mode of discursion presupposes the first mode; for whosoever proceeds from principles to conclusions does not consider both at once; secondly, because to discourse thus is to proceed from the known to the unknown. Hence it is manifest that when the first is known, the second is still unknown; and thus the second is known not in the first, but from the first. Now the term discursive reasoning is attained when the second is seen in the first, by resolving the effects into their causes; and then the discursion ceases. Hence as God sees His effects in Himself as their cause, His knowledge is not discursive.
Reply to Objection 1. Altogether there is only one act of understanding in itself, nevertheless many things may be understood in one (medium), as shown above.
Reply to Objection 2. God does not know by their cause, known, as it were previously, effects unknown; but He knows the effects in the cause; and hence His knowledge is not discursive, as was shown above.
Reply to Objection 3. God sees the effects of created causes in the causes themselves, much better than we can; but still not in such a manner that the knowledge of the effects is caused in Him by the knowledge of the created causes, as is the case with us; and hence His knowledge is not discursive.
Source: Summa Theologica (New Advent)