Thomas Aquinas, along with all the other Church Fathers of the Catholic and Orthodox churches, believed that creation had a beginning. The beginning brought time into being.
Thomas Aquinas also shared the Evangelical Creationist belief that God created many forms of life and the substrate upon which they depend directly; he brought creatures into existence that had not previously existed.
While God’s sustaining of creatures is continuous, their creation refers to their origin in the beginning.
Aquinas’ explanation of creation is that it is to make things by combining form with matter. God created forms according to ideas in his mind, ideas which are perfect exemplars of forms; each idea is a concept and each concept is a word.
The specification of the form is from mind and from nothing; creatures are made out of matter, but matter does not dictate their original form. (Just as writing is in ink, but the ink does not determine what is written on the page).
Thomas Aquinas states that to create something from nothing shows infinite power.
The Principles of Creation 