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. 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... 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 (ST I Q46 A2).In St. Thomas' day, the question was whether the world is eternal. Aristotle affirmed it, as did the Averroists of Aquinas' time (McInerny, Thomas Aquinas: Selected Writings, p. 710f) St. Thomas insisted that the world must have had a beginning, because this is what Genesis 1:1 tells us. But he argues that this is not something that can be known from creation itself. It is interesting to note the last sentence in the quotation above, in terms of its applicability today. Unfortunately unbelieving scientists and others have been given reason to scoff at the Christian faith thanks to those folks suppose that a young earth can be scientifically proven. This gives us a good example as to why it's important to be cautious about what we claim is demonstrable, and what must be accepted on faith. Reason has its limits, and we Christians must accept them no less than others.
[Disclaimer: as a Protestant I was in the creation science camp myself]