Objection 1. It seems that foreknowledge of merits is the cause of predestination. For the Apostle says (Romans 8:29): "Whom He foreknew, He also predestined." Again a gloss of Ambrose on Romans 9:15: "I will have mercy upon whom I will have mercy" says: "I will give mercy to him who, I foresee, will turn to Me with his whole heart." Therefore it seems the foreknowledge of merits is the cause of predestination.
Objection 2. Further, Divine predestination includes the divine will, which by no means can be irrational; since predestination is "the purpose to have mercy," as Augustine says (De Praed. Sanct. ii, 17). But there can be no other reason for predestination than the foreknowledge of merits. Therefore it must be the cause of reason of predestination.
Objection 3. Further, "There is no injustice in God" (Romans 9:14). Now it would seem unjust that unequal things be given to equals. But all men are equal as regards both nature and original sin; and inequality in them arises from the merits or demerits of their actions. Therefore God does not prepare unequal things for men by predestinating and reprobating, unless through the foreknowledge of their merits and demerits.
On the contrary, The Apostle says (Titus 3:5): "Not by works of justice which we have done, but according to His mercy He saved us." But as He saved us, so He predestined that we should be saved. Therefore, foreknowledge of merits is not the cause or reason of predestination.
I answer that, Since predestination includes will, as was said above (Article 4), the reason of predestination must be sought for in the same way as was the reason of the will of God. Now it was shown above (Question 19, Article 5), that we cannot assign any cause of the divine will on the part of the act of willing; but a reason can be found on the part of the things willed; inasmuch as God wills one thing on account of something else. Wherefore nobody has been so insane as to say that merit is the cause of divine predestination as regards the act of the predestinator. But this is the question, whether, as regards the effect, predestination has any cause; or what comes to the same thing, whether God pre-ordained that He would give the effect of predestination to anyone on account of any merits.
Accordingly there were some who held that the effect of predestination was pre-ordained for some on account of pre-existing merits in a former life. This was the opinion of Origen, who thought that the souls of men were created in the beginning, and according to the diversity of their works different states were assigned to them in this world when united with the body. The Apostle, however, rebuts this opinion where he says (Romans 9:11-12): "For when they were not yet born, nor had done any good or evil... not of works, but of Him that calleth, it was said of her: The elder shall serve the younger."
Others said that pre-existing merits in this life are the reason and cause of the effect of predestination. For the Pelagians taught that the beginning of doing well came from us; and the consummaion from God: so that it came about that the effect of predestination was granted to one, and not to another, because the one made a beginning by preparing, whereas the other did not. But against this we have the saying of the Apostle (2 Corinthians 3:5), that "we are not sufficient to think anything of ourselves as of ourselves." Now no principle of action can be imagined previous to the act of thinking. Wherefore it cannot be said that anything begun in us can be the reason of the effect of predestination.
And so others said that merits following the effect of predestination are the reason of predestination; giving us to understand that God gives grace to a person, and pre-ordains that He will give it, because He knows beforehand that He will make good use of that grace, as if a king were to give a horse to a soldier because he knows he will make good use of it. But these seem to have drawn a distinction between that which flows from grace, and that which flows from free will, as if the same thing cannot come from both. It is, however, manifest that what is of grace is the effect of predestination; and this cannot be considered as the reason of predestination, since it is contained in the notion of predestination. Therefore, if anything else in us be the reason of predestination, it will outside the effect of predestination. Now there is no distinction between what flows from free will, and what is of predestination; as there is not distinction between what flows from a secondary cause and from a first cause. For the providence of God produces effects through the operation of secondary causes, as was above shown (22, 3). Wherefore, that which flows from free-will is also of predestination. We must say, therefore, that the effect of predestination may be considered in a twofold light--in one way, in particular; and thus there is no reason why one effect of predestination should not be the reason or cause of another; a subsequent effect being the reason of a previous effect, as its final cause; and the previous effect being the reason of the subsequent as its meritorious cause, which is reduced to the disposition of the matter. Thus we might say that God pre-ordained to give glory on account of merit, and that He pre-ordained to give grace to merit glory. In another way, the effect of predestination may be considered in general. Thus, it is impossible that the whole of the effect of predestination in general should have any cause as coming from us; because whatsoever is in man disposing him towards salvation, is all included under the effect of predestination; even the preparation for grace. For neither does this happen otherwise than by divine help, according to the prophet Jeremias (Lamentations 5:21): "convert us, O Lord, to Thee, and we shall be converted." Yet predestination has in this way, in regard to its effect, the goodness of God for its reason; towards which the whole effect of predestination is directed as to an end; and from which it proceeds, as from its first moving principle.
Reply to Objection 1. The use of grace foreknown by God is not the cause of conferring grace, except after the manner of a final cause; as was explained above.
Reply to Objection 2. Predestination has its foundation in the goodness of God as regards its effects in general. Considered in its particular effects, however, one effect is the reason of another; as already stated.
Reply to Objection 3. The reason for the predestination of some, and reprobation of others, must be sought for in the goodness of God. Thus He is said to have made all things through His goodness, so that the divine goodness might be represented in things. Now it is necessary that God's goodness, which in itself is one and undivided, should be manifested in many ways in His creation; because creatures in themselves cannot attain to the simplicity of God. Thus it is that for the completion of the universe there are required different grades of being; some of which hold a high and some a low place in the universe. That this multiformity of grades may be preserved in things, God allows some evils, lest many good things should never happen, as was said above (Question 22, Article 2). Let us then consider the whole of the human race, as we consider the whole universe. God wills to manifest His goodness in men; in respect to those whom He predestines, by means of His mercy, as sparing them; and in respect of others, whom he reprobates, by means of His justice, in punishing them. This is the reason why God elects some and rejects others. To this the Apostle refers, saying (Romans 9:22-23): "What if God, willing to show His wrath [that is, the vengeance of His justice], and to make His power known, endured [that is, permitted] with much patience vessels of wrath, fitted for destruction; that He might show the riches of His glory on the vessels of mercy, which He hath prepared unto glory" and (2 Timothy 2:20): "But in a great house there are not only vessels of gold and silver; but also of wood and of earth; and some, indeed, unto honor, but some unto dishonor." Yet why He chooses some for glory, and reprobates others, has no reason, except the divine will. Whence Augustine says (Tract. xxvi. in Joan.): "Why He draws one, and another He draws not, seek not to judge, if thou dost not wish to err." Thus too, in the things of nature, a reason can be assigned, since primary matter is altogether uniform, why one part of it was fashioned by God from the beginning under the form of fire, another under the form of earth, that there might be a diversity of species in things of nature. Yet why this particular part of matter is under this particular form, and that under another, depends upon the simple will of God; as from the simple will of the artificer it depends that this stone is in part of the wall, and that in another; although the plan requires that some stones should be in this place, and some in that place. Neither on this account can there be said to be injustice in God, if He prepares unequal lots for not unequal things. This would be altogether contrary to the notion of justice, if the effect of predestination were granted as a debt, and not gratuitously. In things which are given gratuitously, a person can give more or less, just as he pleases (provided he deprives nobody of his due), without any infringement of justice. This is what the master of the house said: "Take what is thine, and go thy way. Is it not lawful for me to do what I will?" (Matthew 20:14-15).
Source: Summa Theologica (New Advent)