Many Jews cited Deuteronomy 6:4 that God is one, but the Hebrew word for one is echad, and it means a combined unity, like the evening and morning made the first day, and again when man cleaved to his wife, they became one flesh–a combined unity of dissimilar entities.
Paul said whatever we can know about God may be understood from the things that are made, even His godhead. So we go back to how things were made in Genesis 1, where the godhead said “Let US make man in our image, after our likeness….