Objection 1. It would seem that it is not an article of faith but a demonstrable conclusion that the world began. For everything that is made has a beginning of its duration. But it can be proved demonstratively that God is the effective cause of the world; indeed this is asserted by the more approved philosophers. Therefore it can be demonstratively proved that the world began.
Objection 2. Further, if it is necessary to say that the world was made by God, it must therefore have been made from nothing or from something. But it was not made from something; otherwise the matter of the world would have preceded the world; against which are the arguments of Aristotle (De Coelo i), who held that heaven was ungenerated. Therefore it must be said that the world was made from nothing; and thus it has being after not being. Therefore it must have begun.
Objection 3. Further, everything which works by intellect works from some principle, as appears in all kinds of craftsmen. But God acts by intellect: therefore His work has a principle. The world, therefore, which is His effect, did not always exist.
Objection 4. Further, it appears manifestly that certain arts have developed, and certain countries have begun to be inhabited at some fixed time. But this would not be the case if the world had been always. Therefore it is manifest that the world did not always exist.
Objection 5. Further, it is certain that nothing can be equal to God. But if the world had always been, it would be equal to God in duration. Therefore it is certain that the world did not always exist.
Objection 6. Further, if the world always was, the consequence is that infinite days preceded this present day. But it is impossible to pass through an infinite medium. Therefore we should never have arrived at this present day; which is manifestly false.
Objection 7. Further, if the world was eternal, generation also was eternal. Therefore one man was begotten of another in an infinite series. But the father is the efficient cause of the son (Phys. ii, text 5). Therefore in efficient causes there could be an infinite series, which is disproved (Metaph. ii, text 5).
Objection 8. Further, if the world and generation always were, there have been an infinite number of men. But man's soul is immortal: therefore an infinite number of human souls would actually now exist, which is impossible. Therefore it can be known with certainty that the world began, and not only is it known by faith.
On the contrary, The articles of faith cannot be proved demonstratively, because faith is of things "that appear not" (Hebrews 11:1). But that God is the Creator of the world: hence that the world began, is an article of faith; for we say, "I believe in one God," etc. And again, Gregory says (Hom. i in Ezech.), that Moses prophesied of the past, saying, "In the beginning God created heaven and earth": in which words the newness of the world is stated. Therefore the newness of the world is known only by revelation; and therefore it cannot be proved demonstratively.
I answer that, By faith alone do we hold, and by no demonstration can it be proved, that the world did not always exist, as was said above of the mystery of the Trinity (32, 1). The reason of this is that the newness of the world cannot be demonstrated on the part of the world itself. For the principle of demonstration is the essence of a thing. Now everything according to its species is abstracted from "here" and "now"; whence it is said that universals are everywhere and always. Hence it cannot be demonstrated that man, or heaven, or a stone were not always. Likewise neither can it be demonstrated on the part of the efficient cause, which acts by will. For the will of God cannot be investigated by reason, except as regards those things which God must will of necessity; and what He wills about creatures is not among these, as was said above (Question 19, Article 3). But the divine will can be manifested by revelation, on which faith rests. Hence that the world began to exist is an object of faith, but not of demonstration or science. And it is useful to consider this, lest anyone, presuming to demonstrate what is of faith, should bring forward reasons that are not cogent, so as to give occasion to unbelievers to laugh, thinking that on such grounds we believe things that are of faith.
Reply to Objection 1. As Augustine says (De Civ. Dei xi, 4), the opinion of philosophers who asserted the eternity of the world was twofold. For some said that the substance of the world was not from God, which is an intolerable error; and therefore it is refuted by proofs that are cogent. Some, however, said that the world was eternal, although made by God. For they hold that the world has a beginning, not of time, but of creation, so that in a certain hardly intelligible way it was always made. "And they try to explain their meaning thus (De Civ. Dei x, 31): for as, if the foot were always in the dust from eternity, there would always be a footprint which without doubt was caused by him who trod on it, so also the world always was, because its Maker always existed." To understand this we must consider that the efficient cause, which acts by motion, of necessity precedes its effect in time; because the effect is only in the end of the action, and every agent must be the principle of action. But if the action is instantaneous and not successive, it is not necessary for the maker to be prior to the thing made in duration as appears in the case of illumination. Hence they say that it does not follow necessarily if God is the active cause of the world, that He should be prior to the world in duration; because creation, by which He produced the world, is not a successive change, as was said above (Question 45, Article 2).
Reply to Objection 2. Those who would say that the world was eternal, would say that the world was made by God from nothing, not that it was made after nothing, according to what we understand by the word creation, but that it was not made from anything; and so also some of them do not reject the word creation, as appears from Avicenna (Metaph. ix, text 4).
Reply to Objection 3. This is the argument of Anaxagoras (as quoted in Phys. viii, text 15). But it does not lead to a necessary conclusion, except as to that intellect which deliberates in order to find out what should be done, which is like movement. Such is the human intellect, but not the divine intellect (14, 7,12).
Reply to Objection 4. Those who hold the eternity of the world hold that some region was changed an infinite number of times, from being uninhabitable to being inhabitable and "vice versa," and likewise they hold that the arts, by reason of various corruptions and accidents, were subject to an infinite variety of advance and decay. Hence Aristotle says (Meteor. i), that it is absurd from such particular changes to hold the opinion of the newness of the whole world.
Reply to Objection 5. Even supposing that the world always was, it would not be equal to God in eternity, as Boethius says (De Consol. v, 6); because the divine Being is all being simultaneously without succession; but with the world it is otherwise.
Reply to Objection 6. Passage is always understood as being from term to term. Whatever bygone day we choose, from it to the present day there is a finite number of days which can be passed through. The objection is founded on the idea that, given two extremes, there is an infinite number of mean terms.
Reply to Objection 7. In efficient causes it is impossible to proceed to infinity "per se"--thus, there cannot be an infinite number of causes that are "per se" required for a certain effect; for instance, that a stone be moved by a stick, the stick by the hand, and so on to infinity. But it is not impossible to proceed to infinity "accidentally" as regards efficient causes; for instance, if all the causes thus infinitely multiplied should have the order of only one cause, their multiplication being accidental, as an artificer acts by means of many hammers accidentally, because one after the other may be broken. It is accidental, therefore, that one particular hammer acts after the action of another; and likewise it is accidental to this particular man as generator to be generated by another man; for he generates as a man, and not as the son of another man. For all men generating hold one grade in efficient causes--viz. the grade of a particular generator. Hence it is not impossible for a man to be generated by man to infinity; but such a thing would be impossible if the generation of this man depended upon this man, and on an elementary body, and on the sun, and so on to infinity.
Reply to Objection 8. Those who hold the eternity of the world evade this reason in many ways. For some do not think it impossible for there to be an actual infinity of souls, as appears from the Metaphysics of Algazel, who says that such a thing is an accidental infinity. But this was disproved above (Question 7, Article 4). Some say that the soul is corrupted with the body. And some say that of all souls only one will remain. But others, as Augustine says [Serm. xiv, De Temp. 4,5; De Haeres., haeres. 46; De Civ. Dei xii. 13, asserted on this account a circuit of souls--viz. that souls separated from their bodies return again thither after a course of time; a fuller consideration of which matters will be given later (75, 2; 118, 6). But be it noted that this argument considers only a particular case. Hence one might say that the world was eternal, or least some creature, as an angel, but not man. But we are considering the question in general, as to whether any creature can exist from eternity.
Source: Summa Theologica (New Advent)