For creation is not a change, but that dependence of the created existence on the principle from which it is instituted, and thus is of the genus of relation; whence nothing prohibits it being in the created as in the subject. Creation is thus said to be a kind of change, according to the way of understanding, insofar as our intellect accepts one and the same thing as not existing before and afterwards existing.

Summa Contra Gentiles II, 18.2 (see also Summa Theologica I, q. 45, art. 3 ad 2)