Aquinas even believed that the beginning of the world in time is impossible to demonstrate:

Article 2. Whether it is an article of faith that the world began?
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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. 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. 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.

​I personally find it difficult to wrap my head around the core argument. The thought seems to be this. Suppose one wanted to demonstrate, for instance, that there was at some time no man, that man was not always. That demonstration would, like any demonstration concerning men, have to proceed from the essence of man. But the essence of man abstracts from "here" and "now"; the essence of man is everywhere and always. One can never draw the consequence from it, then, that man at some time was not; that could not be, as it were, part of the content of the essence of man, from which the demonstration would have to proceed.