The truths about God thus far proposed have been subtly discussed by a number of pagan philosophers, although some of them erred concerning these matters. And those who propounded true doctrine in this respect were scarcely able to arrive at such truths even after long and painstaking investigation. But there are other truths about God revealed to us in the teaching of the Christian religion, which were beyond the reach of the philosopher. These are truths about which we are instructed, in accord with the norm of Christian faith, in a way that transcends human perception. The teaching is this: although God is one and simple, as has been explained, God is Father, God is Son, and God is Holy Spirit. And these three are not three gods, but are one God. We now turn to a consideration of this truth, so far as is possible to us.
St. Thomas Aquinas. (1265-1274). Compendium Theologiae: Philosophical Character of this Doctrine, trans. by Cyril Vollert. St. Louis & London: B. Herder Book Co., 1947
All scripture is given by inspiration of God and is profitable for doctrine, reproof, correction, and instruction in righteousness (2 Timothy 3:16).