Aquinas had a strong view of God’s sovereignty, but also of human free choice. Much of his teaching is rather dense, but one of his books is refreshingly clear. In his book De Rationes Fidei (Reasons for Faith) Thomas answers the question of whether divine predestination eliminates human freedom. Here are excerpts from his answer:
“It is erroneous to say that human acts and events escape God’s fore-knowledge and ordination. It is no less erroneous to say that God’s fore-knowledge and ordination imposes necessity on human acts; otherwise free will would be removed, as well as the value of taking counsel, the usefulness of laws, the care to do what is right and the justice of rewards and punishments. . . We must observe that God knows things differently from man. Man is subject to time and therefore knows things temporally, seeing some things as present, recalling others as past, and fore-seeing others as future. But God is above the passage of time, and his existence is eternal. So his knowledge is not temporal, but eternal. . . Thus God, who looks at everything from the high point of eternity, views as present the whole passage of time and everything that is done in time. Therefore, when I see Socrates sitting, my knowledge is infallible and certain, but no necessity is imposed on Socrates to be seated. Thus God, seeing everything that is past, future or present to us as present to himself, knows all this infallibly and certainly, yet without imposing on contingent things any necessity of existing. . . Just as God’s knowledge does not impose necessity on contingent things, neither does his ordination, by which he providentially orders the universe. For he orders things the way he acts on things; his ordination does not violate but brings to effect by his power what he planned in his Wisdom.As for the action of God’s power, we should observe that he acts in everything and moves each single thing to its actions according to the manner proper to each thing, so that some things, by divine motion, act from necessity, as the motion of heavenly bodies [according to ancient cosmology], while others contingently, which sometimes fail in their proper action because of their corruptibility. A tree, for example, sometimes is impeded from producing fruit and an animal from generating offspring. Thus Divine Wisdom orders things so that they happen after the manner of their proper causes. In the case of man, it is natural for him to act freely, not forced, because rational powers can turn in opposite directions. Thus God orders human actions in a way that these actions are not subject to necessity, but come from free will.”
Thus Thomas Aquinas holds that God can have certain knowledge of how everything will be, past and future, and God can ordain how things will be, even to the point of causing these things to come to pass, but can do all this without violating human free choice. God works through human free will, not as a puppet master, but similarly to how God causes a rose to bloom: He works through the nature of a rose. Likewise, it is the nature of man to have a free will, therefore God works through man’s will to accomplish His purposes, and bring about the certain conclusions of His plan, not those of mankind.