3 But as it is the nature and endeavour of the good and wise to prepare themselves wholly for securing either the reality or the opportunity of some precious hope lest their preparedness should in some respects fall short of that which they look for—so in like manner those who are filled with the madness of heretical frenzy make it their chiefest anxiety to labour with all the ingenuity of their impiety against the truth of pious faith, in order that against those who are religious they may establish their own irreligion; that they may surpass the hope of our life in the hopelessness of their own, and that they may spend more thought over false than we spend over true teaching.
For against the pious assertions of our faith they have carefully devised such objections of their impious misbelief, as first to ask whether we believe in one God, next, whether Christ also be God, lastly, whether the Father is greater than the Son, in order that when they hear us confess that God is one they may use our reply to show that Christ cannot be God. For they do not enquire concerning the Son whether He be God; all they wish for in asking questions about Christ is to prove that He is not a Son, that by entrapping men of simple faith they may through the belief in one God divert them from the belief in Christ as God, on the ground that God is no longer one if Christ also must be acknowledged as God.
Again with what subtlety of worldly wisdom do they contend when they say, If God is one, whosoever that other shall be shown to be, he will not be God. For if there be another God He can no longer be one, since nature does not permit that where there is another there should be one only, or that where there is only one there should be another. Afterwards, when by the crafty cunning of this insidious argument they have misled those who are ready to believe and listen, they then apply this proposition (as if they could now establish it by an easier method), that Christ is God rather in name than in nature, because this generic name in Him can destroy in none that only true belief in one God: and they contend that through this the Father is greater than the Son, because, the natures being different, as there is but one God, the Father is greater from the essential character of His nature; and that the Other is only called Son while He is really a creature subsisting by the will of the Father, because He is less than the Father; and also that He is not God, because God being one does not admit of another God, since he who is less must necessarily be of a nature alien from that of the person who is greater. Again, how foolish they are in their attempts to lay down a law for God when they maintain that no birth can take place from one single being, because throughout the universe birth arises from the union of two; moreover, that the unchangeable God cannot accord from Himself birth to one who is born, because that which is changeless is incapable of addition, nor can the nature of a solitary and single being contain within itself the property of generation.
Source: On the Trinity (New Advent)