"There is a twofold mode of truth in what we profess about God. Some truths about God exceed all the ability of the human reason. Such is the truth that God is triune. But there are some truths which the natural reason also is able to reach. Such are that God exists, that He is one, and the like. In fact, such truths about God have been proved demonstratively by the philosophers, guided by the light of the natural reason" (Summa Contra Gentiles 1.3).
"Est autem in his quae de Deo confitemur duplex veritatis modus. Quaedam namque vera sunt de Deo quae omnem facultatem humanae rationis excedunt, ut Deum esse trinum et unum. Quaedam vero sunt ad quae etiam ratio naturalis pertingere potest, sicut est Deum esse, Deum esse unum, et alia huiusmodi; quae etiam philosophi demonstrative de Deo probaverunt, ducti naturalis lumine rationis."
Translation by Anton C. Pegis.