Objection 1. It seems that the knowledge of God is variable. For knowledge is related to what is knowable. But whatever imports relation to the creature is applied to God from time, and varies according to the variation of creatures. Therefore the knowledge of God is variable according to the variation of creatures.
Objection 2. Further, whatever God can make, He can know. But God can make more than He does. Therefore He can know more than He knows. Thus His knowledge can vary according to increase and diminution.
Objection 3. Further, God knew that Christ would be born. But He does not know now that Christ will be born; because Christ is not to be born in the future. Therefore God does not know everything He once knew; and thus the knowledge of God is variable.
On the contrary, It is said, that in God "there is no change nor shadow of alteration" (James 1:17).
I answer that, Since the knowledge of God is His substance, as is clear from the foregoing (4), just as His substance is altogether immutable, as shown above (Question 9, Article 1), so His knowledge likewise must be altogether invariable.
Reply to Objection 1. "Lord", "Creator" and the like, import relations to creatures in so far as they are in themselves. But the knowledge of God imports relation to creatures in so far as they are in God; because everything is actually understood according as it is in the one who understands. Now created things are in God in an invariable manner; while they exist variably in themselves. We may also say that "Lord", "Creator" and the like, import the relations consequent upon the acts which are understood as terminating in the creatures themselves, as they are in themselves; and thus these relations are attributed to God variously, according to the variation of creatures. But "knowledge" and "love," and the like, import relations consequent upon the acts which are understood to be in God; and therefore these are predicated of God in an invariable manner.
Reply to Objection 2. God knows also what He can make, and does not make. Hence from the fact that He can make more than He makes, it does not follow that He can know more than He knows, unless this be referred to the knowledge of vision, according to which He is said to know those things which are in act in some period of time. But from the fact that He knows some things might be which are not, or that some things might not be which are, it does not follow that His knowledge is variable, but rather that He knows the variability of things. If, however, anything existed which God did not previously know, and afterwards knew, then His knowledge would be variable. But this could not be; for whatever is, or can be in any period of time, is known by God in His eternity. Therefore from the fact that a thing exists in some period of time, it follows that it is known by God from eternity. Therefore it cannot be granted that God can know more than He knows; because such a proposition implies that first of all He did not know, and then afterwards knew.
Reply to Objection 3. The ancient Nominalists said that it was the same thing to say "Christ is born" and "will be born" and "was born"; because the same thing is signified by these three--viz. the nativity of Christ. Therefore it follows, they said, that whatever God knew, He knows; because now He knows that Christ is born, which means the same thing as that Christ will be born. This opinion, however, is false; both because the diversity in the parts of a sentence causes a diversity of enunciations; and because it would follow that a proposition which is true once would be always true; which is contrary to what the Philosopher lays down (Categor. iii) when he says that this sentence, "Socrates sits," is true when he is sitting, and false when he rises up. Therefore, it must be conceded that this proposition is not true, "Whatever God knew He knows," if referred to enunciable propositions. But because of this, it does not follow that the knowledge of God is variable. For as it is without variation in the divine knowledge that God knows one and the same thing sometime to be, and sometime not to be, so it is without variation in the divine knowledge that God knows an enunciable proposition is sometime true, and sometime false. The knowledge of God, however, would be variable if He knew enunciable things by way of enunciation, by composition and division, as occurs in our intellect. Hence our knowledge varies either as regards truth and falsity, for example, if when either as regards truth and falsity, for example, if when a thing suffers change we retained the same opinion about it; or as regards diverse opinions, as if we first thought that anyone was sitting, and afterwards thought that he was not sitting; neither of which can be in God.
Source: Summa Theologica (New Advent)