3 And so the arrangement of our treatise follows closely the order of the objections raised. Since the next article of their blasphemous and dishonest confession is, We confess One true God, the whole of this second book is devoted to the question whether the Son of God be true God. For it is clear that the heretics have ingeniously contrived this arrangement of first naming One God and then One true God, in order to detach the Son from the name and nature of God; since the thought must suggest itself that, truth being inherent in the One God, it must be strictly confined to Him.
And therefore, since it is clear beyond a doubt that Moses, when he proclaimed the unity of God, meant therein to assert the Divinity of the Son, let us return to the leading passages in which his teaching is conveyed, and enquire whether or no he wishes us to believe that the Son, Who, as he has taught us, is God, is also true God. It is clear that the truth, or genuineness, of a thing is a question of its nature and its powers. For instance, true wheat is that which grows to a head with the beard bristling round it, which is purged from the chaff and ground to flour, compounded into a loaf and taken for food, and renders the nature and the uses of bread.
Thus natural powers are the evidence of truth; and let us see, by this test, whether He, Whom Moses calls God, be true God. We will defer for the present our discourse concerning this One God, Who is also true God, lest, if I fail at once to take up their challenge and uphold the One True God in the two Persons of Father and of Son, eager and anxious souls be oppressed by dangerous doubts.
Source: On the Trinity (New Advent)