Reply Obj. 10: Given the action, the effect follows according to the
requirement of the form, which is the principle of action. But in
agents acting by will, what is conceived and preordained is to be
taken as the form, which is the principle of action. Therefore from
the eternal action of God an eternal effect did not follow; but such
an effect as God willed, an effect, to wit, which has being after not
being.
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SECOND ARTICLE [I, Q. 46, Art. 2]

Whether It Is an Article of Faith That the World Began?

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.

Obj. 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.

Obj. 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.

Obj. 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.

Obj. 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.

Obj. 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.

Obj. 7: Further, if the world was eternal, generation also was
eternal. Therefore one man was begott