Objection 2: Further, no reason can be assigned why God pre-ordains to salvation one number of men more than another. But nothing is arranged by God without a reason. Therefore the number to be saved pre-ordained by God cannot be certain.
Objection 3: Further, the operations of God are more perfect than those of nature. But in the works of nature, good is found in the majority of things; defect and evil in the minority. If, then, the number of the saved were fixed by God at a certain figure, there would be more saved than lost. Yet the contrary follows from Mat.7:13,14: "For wide is the gate, and broad the way that leadeth to destruction, and many there are who go in thereat. How narrow is the gate, and strait is the way that leadeth to life; and few there are who find it!" Therefore the number of those pre-ordained by God to be saved is not certain.
On the contrary, Augustine says (De Corr. et Grat.13): "The number of the predestined is certain, and can neither be increased nor diminished."
I answer that, The number of the predestined is certain. Some have said that it was formally, but not materially certain; as if we were to say that it was certain that a hundred or a thousand would be saved; not however these or those individuals. But this destroys the certainty of predestination; of which we spoke above (A). Therefore we must say that to God the number of the predestined is certain, not only formally, but also materially. It must, however, be observed that the number of the predestined is said to be certain to God, not by reason of His knowledge, because, that is to say, He knows how many will be saved (for in this way the number of drops of rain and the sands of the sea are certain to God); but by reason of His deliberate choice and determination. For the further evidence of which we must remember that every agent intends to make something finite, as is clear from what has been said above when we treated of the infinite (Q, AA,3). Now whosoever intends some definite measure in his effect thinks out some definite number in the essential parts, which are by their very nature required for the perfection of the whole. For of those things which are required not principally, but only on account of something else, he does not select any definite number "per se"; but he accepts and uses them in such numbers as are necessary on account of that other thing. For instance, a builder thinks out the definite measurements of a house, and also the definite number of rooms which he wishes to make in the house; and definite measurements of the walls and roof; he does not, however, select a definite number of stones, but accepts and uses just so many as are sufficient for the required measurements of the wall. So also must we consider concerning God in regard to the whole universe, which is His effect. For He pre-ordained the measurements of the whole of the universe, and what number would befit the essential parts of that universe -- -that is to say, which have in some way been ordained in perpetuity; how many spheres, how many stars, how many elements, and how many species. Individuals, however, which undergo corruption, are not ordained as it were chiefly for the good of the universe, but in a secondary way, inasmuch as the good of the species is preserved through them. Whence, although God knows the total number of individuals, the number of oxen, flies and such like, is not pre-ordained by God "per se"; but divine providence produces just so many as are sufficient for the preservation of the species. Now of all creatures the rational creature is chiefly ordained for the good of the universe, being as such incorruptible; more especially those who attain to eternal happiness, since they more immediately reach the ultimate end. Whence the number of the predestined is certain to God; not only by way of knowledge, but also by way of a principal pre-ordination.
It is not exactly the same thing in the case of the number of the reprobate, who would seem to be pre-ordained by God for the good of the elect, in whose regard "all things work together unto good" (Rom.8:28). Concerning the number of all the predestined, some say that so many men will be saved as angels fell; some, so many as there were angels left; others, as many as the number of angels created by God. It is, however, better to say that, "to God alone is known the number for whom is reserved eternal happiness [*From the 'secret' prayer of the missal, 'pro vivis et defunctis.']"
Reply to Objection 1: These words of Deuteronomy must be taken as applied to those who are marked out by God beforehand in respect to present righteousness. For their number is increased and diminished, but not the number of the predestined.
Reply to Objection 2: The reason of the quantity of any one part must be judged from the proportion of that part of the whole. Thus in God the reason why He has made so many stars, or so many species of things, or predestined so many, is according to the proportion of the principal parts to the good of the whole universe.
Reply to Objection 3: The good that is proportionate to the common state of nature is to be found in the majority; and is wanting in the minority. The good that exceeds the common state of nature is to be found in the minority, and is wanting in the majority. Thus it is clear that the majority of men have a sufficient knowledge for the guidance of life; and those who have not this knowledge are said to be half-witted or foolish; but they who attain to a profound knowledge of things intelligible are a very small minority in respect to the rest. Since their eternal happiness, consisting in the vision of God, exceeds the common state of nature, and especially in so far as this is deprived of grace through the corruption of original sin, those who are saved are in the minority. In this especially, however, appears the mercy of God, that He has chosen some for that salvation, from which very many in accordance with the common course and tendency of nature fall short.