Reginald Garrigou-Lagrange writes,
“[T]he only way a finite cause can produce its effect is by transforming an already existing object capable of such transformation. Thus a sculptor, in order to carve his statue, requires a material; so also a teacher gradually forms the intelligence of his pupil, but he did not give him intelligence.
“The greater the poverty of the object to be transformed, the greater must be the wealth and fecundity of the transforming active power.” 
But what if the poverty of the thing is so extreme that it lacks even existence? What then must be the wealth and fecundity of the transforming active power?
St. Thomas says,
“[I]f a greater power is required in the agent in proportion to the distance of the potentiality from the act, it follows that the power of that which produces something from no presupposed potentiality is infinite, because there is no proportion between ‘no potentiality’ and the potentiality presupposed by the power of a natural agent, as there is no proportion between ‘not being’ and ‘being.’” 
The counterpart, if you will, of a strong thing is a weak thing; and the weaker the thing, the stronger needs to be the agent that can transform it into the thing desired. Thus, there is a proportion between the weakness of the thing and the strength of the transforming agent.
But what proportion can there be between a transforming agent and nothing? There can be none. Being and non-being are entirely disproportionate. You can’t say how much strength corresponds to the weakness of nothing, because the weakness of nothing is absolute. The strength of its counterpart, therefore, must also be absolute, or in other words infinite. Thus, the power needed to make something out of nothing, is infinite power.
“Not only was it impossible for even the most exalted angel to create the physical universe, but he cannot create so much as a speck of dust; and it will ever be so. To create anything out of nothing—that is, without any pre-existing subject whatever—an infinite power is required.” 
 Reginald Garrigou-Lagrange, Providence (1932), Part II, Chapter 8.
 Summa Theologica I.I., Q. 45, A. 5.