Thomas Aquinas on the Trinity in Genesis 1

"[One reason] why the knowledge of the divine persons [that is, that God is triune] was necessary for us . . . [is that i]t was necessary for the right idea of creation. The fact of saying that God made all things by His Word excludes the error of those who say that God produced things by necessity. When we say that in Him there is a procession of love, we show that God produced creatures not because He needed them, nor because of any other extrinsic reason, but on account of the love of His own goodness. So Moses, when he had said, In the beginning God created heaven and earth, he subjoined, God said, Let there be light, to manifest the divine Word; and then said, God saw the light that it was good, to show the proof of the divine love [that is, the Holy Spirit]. The same is also found in the other works of creation."

—Thomas Aquinas, Summa Theologica, Q32 a1 ad3