35 First of all, then, O heretic that hast no part in the Spirit which spoke by the Apostle, learn your folly. If you wrongly employest the confession of one God to deny the Godhead of Christ, on the ground that where one God exists He must be regarded as solitary, and that to be One is characteristic and peculiar to Him Who is One—what sense will you assign to the statement that Jesus Christ is one Lord? For if, as you assert, the fact that the Father alone is God has not left to Christ the possibility of Godhead, it must needs be also according to you that the fact of Christ being one Lord does not leave God the possibility of being Lord, seeing that you will have it that to be One must be the essential property of Him Who is One. Hence if you deny that the one Lord Christ is also God, you must needs deny that the one God the Father is also Lord. And what will the greatness of God amount to if He be not Lord, and the power of the Lord if He be not God: since it (viz., the greatness or power) causes that to be God which is Lord, and makes that Lord which is God?
Source: On the Trinity (New Advent)