2 The reason why I have just mentioned this utterance of the Apostle is this; men of crooked minds and false professions, void of hope and venomous of speech, lay upon me the necessity of inveighing against them, because under the guise of religion they instil deadly doctrines, infectious thoughts and corrupt desires into the simple minds of their hearers. And this they do with an utter disregard of the true sense of the apostolic teaching, so that the Father is not a Father, nor the Son, Son, nor the Faith, the Faith.
In resisting their wild falsehoods, we have extended the course of our reply so far, that after proving from the Law that God and God were distinct and that very God was in very God, we then showed from the teaching of evangelists and apostles the perfect and true birth of the Only-begotten God; and lastly, we pointed out in the due course of our argument that the Son of God is very God, and of a nature identical with the Father's, so that the faith of the Church should neither confess that God is single nor that there are two Gods.
For neither would the birth of God allow God to be solitary, nor would a perfect birth allow different natures to be ascribed to two Gods. Now in refuting their vain speaking we have a twofold object, first that we may teach what is holy and perfect and sound, and, that our discourse should not by straying through any by-paths and crooked ways, and struggling out of devious and winding tunnels, seem rather to search for the truth than declare it. Our second object is that we should reveal to the conviction of all men the folly and absurdity of those crafty arguments of their vain and deceitful opinions which are adapted to a plausible show of seductive truth.
For it is not enough for us to have pointed out what things are good, unless they are understood to be absolutely good by our refutation of their opposites.
Source: On the Trinity (New Advent)