5 When the Law says, And God said, Let there be a firmament, and then adds, And God made the firmament, it introduces no other distinction than that of Person. It indicates no difference of power or nature, and makes no change of name. Under the one title of God it reveals, first, the thought of Him Who spoke, and then the action of Him Who created. The language of the narrator says nothing to deprive Him of Divine nature and power; nay rather, how precisely does it inculcate His true Godhead.
The power to give effect to the word of creation belongs only to that Nature with Whom to speak is the same as to fulfil. How then is He not true God, Who creates, if He is true God, Who commands? If the word spoken was truly Divine, the deed done was truly Divine also. God spoke, and God created; if it was true God Who spoke, He Who created was true God also; unless indeed, while the presence of true Godhead was displayed in the speech of the One, its absence was manifested in the action of the Other.
Thus in the Son of God we behold the true Divine nature. He is God, He is Creator, He is Son of God, He is omnipotent. It is not merely that He can do whatever He will, for will is always the concomitant of power; but He can do also whatever is commanded Him. Absolute power is this, that its possessor can execute as Agent whatever His words as Speaker can express. When unlimited power of expression is combined with unlimited power of execution, then this creative power, commensurate with the commanding word, possesses the true nature of God.
Thus the Son of God is not false God, nor God by adoption, nor God by gift of the name, but true God. Nothing would be gained by the statement of the arguments by which His true Godhead is opposed. His possession of the name and of the nature of God is conclusive proof. He, by Whom all things were made, is God. So much the creation of the world tells me about Him. He is God, equal with God in name; true God, equal with true God in power. The might of God is revealed to us in the creative word; the might of God is manifested also in the creative act. And now again I ask by what authority you deny, in your confession of Father and Son, the true Divine nature of Him Whose name reveals His power, Whose power proves His right to the Name.
Source: On the Trinity (New Advent)