“I answer that, As said above (Question 44, Article 2), we must consider not only the emanation of a particular being from a particular agent, but also the emanation of all being from the universal cause, which is God; and this emanation we designate by the name of creation. Now what proceeds by particular emanation, is not presupposed to that emanation; as when a man is generated, he was not before, but man is made from ‘not-man,’ and white from ‘not-white.’ Hence if the emanation of the whole universal being from the first principle be considered, it is impossible that any being should be presupposed before this emanation. For nothing is the same as no being. Therefore as the generation of a man is from the ‘not-being’ which is ‘not-man,’ so creation, which is the emanation of all being, is from the ‘not-being’ which is ‘nothing.'”
“[W]hat is created, is not made by movement, or by change. For what is made by movement or by change is made from something pre-existing. And this happens, indeed, in the particular productions of some beings, but cannot happen in the production of all being by the universal cause of all beings, which is God.”
St. Thomas is arguing, basically, that if God is the source of all being, then he had to have created ex nihilo. If he created from preexisting matter, then matter is a being which does not have God as its source. This holds true whether you believe that our universe is all that exists, or that ours is but one universe of a multiverse. In other words, even if you believe God is only God of our universe, he nevertheless can’t be the source of all being within even our own universe, if he did not create our universe ex nihilo.
Some believe God didn’t create ex nihilo, but merely formed things out of preexisting matter. But we know that all material things are formed from natural processes, going all the way back to the Big Bang. Even the “heavy” elements of which we and the earth are made are known to have been formed from the lighter elements hydrogen, helium and lithium, through the life cycles of stars.
So if God merely formed everything that exists, it seems that can only mean that he took a bunch of matter, compressed it into the singularity that existed just prior to the Big Bang, and somehow designed and shaped it such that after it exploded, it would form into galaxies, stars and planets, on which life would evolve — presumably according to a plan of his.
In which case the only difference between us, is that we believe God is the source of all the matter/energy that was compressed into the singularity, whereas those who deny creation ex nihilo contend that God took preexisting stuff and formed it into the singularity.
It has been argued (e.g. here) that God’s not being infinite (eternal, all-powerful, all-knowing, omnipresent) goes a long way towards solving the problem of evil. If he is not the source of all that exists and doesn’t have absolute power over it at every moment, then he can’t be held responsible for the evil that occurs. He does the best he can within certain constraints, but can’t do anything and everything he might want to do.
But even if God is not the source of all being, i.e. if matter did not come from him but is eternally preexisting, nevertheless, if he could form matter/energy into the singularity and guide and direct it such that it formed galaxies, stars and planets, and life evolved, all from natural causes — to do all that, wouldn’t he have to be effectively infinite? Wouldn’t he have to know pretty much everything about the universe, and have power over pretty much everything in it, in order to accomplish such a feat?
Whereas if he did not form matter into the singularity such that all things would develop and evolve as they have, and knowing and intending that they would do so, then it’s hard to know in what sense he may be called the Creator.