Summa Theologiae I-II, Q112, a2, and SCG III 160-162
 
Is preparation for grace required on man’s part? This is one of the rare articles in which Thomas answers both Yes and No. Grace can be understood in either of two ways: as the habitual gift of God (i.e., the infused virtues of faith, hope, and charity) or as supernatural help from God in general—i.e., any movement of the human soul toward the good. For grace in the first sense, some preparation is required, since the “forms” of the supernatural virtues can only come into “matter” that is suitably prepared for them. Thomas has in mind some act of consent on the part of the human being toward receiving grace, since God does not infuse the supernatural virtues into someone against his will.

In the second sense, no preparation is possible, since no movement of the soul toward God can occur without God’s help. Even the act of free-will by which we consent to the reception of the supernatural virtues is something worked in us by God.
 
Article 3. Is grace necessarily given to everyone who prepares for it?

Yes, but only because of God’s infallible concordance with His own decrees. Thomas considers an argument (in objection 3) based on Dionysius’s principle that good is naturally self-diffusive. Since grace is a higher good than any good of nature, it should be supremely self-diffusive, and so it should “automatically” flow into anyone whose heart is suitably prepared for it. In response, Thomas points out that the diffusion of good requires the action of an agent—a suitably prepared piece of matter is not automatically elevated to a higher state. And, since God is the agent and God acts freely in creating, the diffusion of grace depends on His free decrees. The self-diffusiveness of good does not require that God give every good to every possible creature.
 
Article 4: Is grace greater in one than in another?
 
Grace can be measured in two ways: in terms of the good to which it directs us, and in terms of the degree to which we receive the influence of the direction (subjectively). All grace is equal in the first sense, since it orders all to the supreme good of the beatific vision. In the second respect, grace can vary from person to person. The better one has prepared oneself for grace, the greater its influence in one’s life. But this too depends on the degree to which we are helped by God to prepare ourselves for grace. Hence, the fundamental source of inequality lies in God’s free choice.

​Summa Theologiae I, Q23, and SCG III.163
 
It is only the elect who are predestined, and they are predestined only for their ultimate end, the beatific vision. Thomas takes the term ‘predestine’ to refer to God’s aiding and directing men to their supernatural end, an end that they cannot achieve by their natural powers alone. Predestination then is simply one aspect of God’s providential care of all things. In reply 3, Thomas clarifies that predestination applies both to elect men and to elect angels, since in both cases the ultimate end exceeds their natural powers.
 
In article 2, Thomas explains that predestination exists in an active way in the mind of God. There is no quality of predestination in the elect. The predestination of the elect consists simply in the fact that they have been chosen for eternal life by God. The execution of predestination occurs through the government of God, which consists in a kind of calling and justifying of the elect by God (Romans 8:30).
 
The opposite of predestination is reprobation (article 3). God is said to reprobate those who are not elect. Reprobation is an instance of God’s permissive will, and an aspect of His providence. God permits some men to fall away from their final end. This reprobation is not mere foreknowledge—it is an aspect of God’s will, “the will to permit (voluntatem permittendi) a person to fall into sin, and to impose the punishment of damnation on account of that sin.”
 
Reply to objection 1: “God loves all men and all creatures, inasmuch as He wishes them all some good; but He does not wish (vult – volo seems to imply a wanting that is somewhat stronger than a mere “wish”) every good to them all. So far, therefore, as He does not wish this particular good—namely eternal life—He is said to hate or reprobate them.”
 
At the same time, Thomas insists (in reply 2) that reprobation is not the cause of sin, as predestination is the cause of grace. “Guilt proceeds from the free-will of the person who is reprobated and deserted by grace.”
 
In objection 3, Thomas considers the question of whether the reprobate are unable to obtain grace. Thomas insists that any such impossibility is not absolute, but only conditional or suppositional. Such conditional necessity does not do away with the liberty of choice. This conditional or suppositional necessity refers only to the immutability of God in His eternal knowledge and will—it means only that each future fact necessitates itself, in the sense that it is necessary that if it will be, it will be.
 
In article 4, Thomas explains that predestination is moved by love for the person and preceded causally by God’s choosing to give them eternal life. It is not a mere knowledge of something independent of God’s will. This point is clarified still further in article 5: predestination does not consist in God’s foreknowledge of the merits of the elect. That is, I am not predestined because God foreknew that I would freely cooperate with God’s grace. The causal relation goes in the opposite direction: I freely cooperate with God’s grace because He elected and predestined me.
 
Thomas takes it as obvious that predestination as a choice existing in God could not be caused by some external factor, like our choices. Nonetheless, we can look for a causal or explanatory order among the things that are willed by God. Consequently, we can ask whether God willed that p because He willed that q, or vice versa, or God willed neither because of the other.  So, we can sensibly ask whether God willed our predestination because He willed our future cooperation, or vice versa.
 
Thomas thinks that those who suppose that predestination follows from cooperation rely on a false dichotomy between grace and free-will, as though what we do by grace we cannot do freely, and vice versa. On this picture, God supplies equal grace to everyone, and then we, totally apart from grace, decide whether to cooperate with it or not. If we decide to cooperate, then God predestines us. Thomas thinks this is based on a false premise. Free cooperation with grace is itself a manifestation of grace. Therefore, predestination causes the future grace, which causes the free cooperation. That which is through (est per) free-will is also of (ex) predestination.
 
Whatever is in a man disposing him toward salvation, even our preparation for grace, is included under the effect of predestination. Lamentations 5:20, “Convert us, O Lord, to Thee, and we shall be converted.”
 
Why then are some predestined and others reprobated? (Objection 3) The reason must be sought in the goodness of God. The completion of the universe, as an expression of God’s glory, requires a variety of grades of being. (Romans 9:22, 23 and II Timothy 2:20) God would be acting unjustly if He owed predestination to all of us as a debt, but instead He gives eternal life gratuitously. He is free to give more or less, as He chooses. (Matthew 20:14, 15)
 
Does predestination take away our choice, making our salvation or damnation necessary, pre-determined? Thomas denies this in article 6, for reasons already given in relation to God’s providence in general. Predestination is an aspect of God’s providence, and His providence does not impose necessity on events. He works through secondary causes, and the working of these causes remains contingent and undetermined. Here Thomas could have appealed to Boethius’s insight: that God is outside of time, and so He cannot properly be said to “fore”-know or “pre”-destine things. He knows and wills from eternity, and eternally His willing and knowing my present and future choices is fully compatible with those choices remaining contingent until I have actually made them. My will is free and undetermined because God’s will is free and undetermined, and God wills what I shall will through my exercise of my own free will.
 
As I argued above, the viability of this answer depends on denying that God has a single, fully unified plan for creation that was in place at the first moment. If God has multiple plans that go into effect at different points in time, it makes sense to ask when someone has become elect or reprobate (i.e., when the plan of God’s that aims or fails to aim at their salvation has come into effect). Clearly, we are all elect (or reprobate) at some point before the last moment of our earthly lives, but how far in advance? I think it may vary from person to person. Once someone is elected, their salvation is no longer contingent (and so no longer up to them). This does not mean that we can know that we are elect before our earthly lives are over: this is impossible, barring some special, private revelation.
 
In Ephesians 1:4, St. Paul writes, “Before the foundation of the world He chose us in Christ to be His people… and He predestined us to be adopted as His children through Jesus Christ.” This does suggest that each elect person was elected from the moment of the world’s creation. However, as Karl Barth pointed out, it is not clear whether Paul is talking here about our individual election or about the collective election of the Church, Christ’s body. The emphasis on “in Christ” and “through Jesus Christ” supports the latter interpretation somewhat. I think that it is possible for our individual election to occur within time, even after our lives have largely unfolded.
 
Once we are elected, our salvation would seem to be no longer contingent and so no longer up to us. However, this does not mean that we would not still have a very great scope of freedom. The degree of glory we will eventually receive might still remain largely contingent.
 
All of this is compatible with God’s having (in eternity) certain and infallible knowledge of the whole course of history in a single, unified act of vision.
 
In article 7, Thomas affirms that the number of the elect is certain. That is, God does not leave the division of elect and reprobate up to chance. The distribution of salvation and damnation serves the good of the creation as a whole. This depends, I think, on St. Thomas’s assumption of a single, unified plan for all of history.
 
In a similar way, for St. Thomas, it was not a matter of chance that Jesus and Mary remained sinless in their earthly lives, but it was nonetheless a matter of their own free choice that they did so. The Blessed Virgin represents the hardest case for a van-Inwagen-style analysis of freedom. Her perfect sinlessness was certainly fixed by God’s plan at the very moment of her conception, if not much earlier. It would seem then that her refraining from sin was never contingent at any moment of her life. How then could she be said to refrain from sinning freely? St. Thomas would reply that her free choices were never pre-determined by any creaturely conditions, but only by God’s sovereign election. Perhaps I could argue that the Blessed Virgin’s sinlessness was always supremely fitting but not (at least not at first) strictly entailed by God’s initial plans for her. He gave her sufficient grace to avoid sin at each point in her life, but it remained for a long time contingent whether or not He would do so. Once again, this is not to call into question whether her sinlessness was grasped eternally by God’s perfect omniscience in a single act of knowing and willing.
 
Jesus, I think, may be a special case. As the Incarnation of God, Jesus enjoys the per se freedom of the divine nature, in the sense that nothing could determine that nature to be what it is or to do what it does. Hence, Jesus’ freedom would be compatible with the complete necessity of His sinlessness.