Some theologians have said that Aquinas’ argument is not dependent on a past, but only on a present cause of being. It is true that act is a continuous present. God is the continuous present of creation. What is made is maintained in actuality by God’s own existence.
Aquinas writes that creatures exist and live through their participation in God. This is one truth, but it does not discount the truth that God conferred existence upon them in the first acts of creation that brought their forms into existence.
Therefore created things had a beginning as well as continuing existence.
The Principles of Creation 