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“In all ordered efficient causes, the first is the cause of the intermediate and the intermediate the cause of the last, whether there be one or many intermediates. The cause being removed, that of which it is the cause is removed. But if in efficient causes there is an infinite regress, none of the causes will be first. Therefore all the others are removed, since they are intermediates. But this is manifestly false. Therefore we must recognize that there is a first efficient cause that is God.”
-Summa contra Gentiles, Book I, Ch. 13


“We have shown above that there is one first being possessing the full perfection of existence whom we call God and who from the abundance of his perfection grants being to all existing things. Thus not only is he the first of beings but the principle of all others. He grants existence to other things, not out of any necessity of nature, but of his free will, as is clear from the foregoing. The result is that he is the Lord of all things he has made, for he rules over what is subject to his will. He has perfect dominion over the things produced by him, since in producing them he requires no external agent nor foundation of matter, since he is the cause of universal being.”
-Summa contra Gentiles, Book III, Ch. 1


“Is God simple?
It seems that he is not.
From one simple thing only one thing comes, for a thing causes something the same as itself, according to the Philosopher. But a multitude of things proceeds from God. Therefore he is not simple.”
-On the Divine Simplicity, Question 7, Article 1