Objection 1. It seems that the knowledge of God is not the cause of things. For Origen says, on Romans 8:30, "Whom He called, them He also justified," etc.: "A thing will happen not because God knows it as future; but because it is future, it is on that account known by God, before it exists."
Objection 2. Further, given the cause, the effect follows. But the knowledge of God is eternal. Therefore if the knowledge of God is the cause of things created, it seems that creatures are eternal.
Objection 3. Further, "The thing known is prior to knowledge, and is its measure," as the Philosopher says (Metaph. x). But what is posterior and measured cannot be a cause. Therefore the knowledge of God is not the cause of things.
On the contrary, Augustine says (De Trin. xv), "Not because they are, does God know all creatures spiritual and temporal, but because He knows them, therefore they are."
I answer that, The knowledge of God is the cause of things. For the knowledge of God is to all creatures what the knowledge of the artificer is to things made by his art. Now the knowledge of the artificer is the cause of the things made by his art from the fact that the artificer works by his intellect. Hence the form of the intellect must be the principle of action; as heat is the principle of heating. Nevertheless, we must observe that a natural form, being a form that remains in that to which it gives existence, denotes a principle of action according only as it has an inclination to an effect; and likewise, the intelligible form does not denote a principle of action in so far as it resides in the one who understands unless there is added to it the inclination to an effect, which inclination is through the will. For since the intelligible form has a relation to opposite things (inasmuch as the same knowledge relates to opposites), it would not produce a determinate effect unless it were determined to one thing by the appetite, as the Philosopher says (Metaph. ix). Now it is manifest that God causes things by His intellect, since His being is His act of understanding; and hence His knowledge must be the cause of things, in so far as His will is joined to it. Hence the knowledge of God as the cause of things is usually called the "knowledge of approbation."
Reply to Objection 1. Origen spoke in reference to that aspect of knowledge to which the idea of causality does not belong unless the will is joined to it, as is said above.
But when he says the reason why God foreknows some things is because they are future, this must be understood according to the cause of consequence, and not according to the cause of essence. For if things are in the future, it follows that God knows them; but not that the futurity of things is the cause why God knows them.
Reply to Objection 2. The knowledge of God is the cause of things according as things are in His knowledge. Now that things should be eternal was not in the knowledge of God; hence although the knowledge of God is eternal, it does not follow that creatures are eternal.
Reply to Objection 3. Natural things are midway between the knowledge of God and our knowledge: for we receive knowledge from natural things, of which God is the cause by His knowledge. Hence, as the natural objects of knowledge are prior to our knowledge, and are its measure, so, the knowledge of God is prior to natural things, and is the measure of them; as, for instance, a house is midway between the knowledge of the builder who made it, and the knowledge of the one who gathers his knowledge of the house from the house already built.
Source: Summa Theologica (New Advent)