6 For they do not receive from the Father the knowledge of the Son; neither do they learn who the Father is from the Son, who teaches clearly and without parables Him who truly is God. He says: “Swear not at all; neither by heaven, for it is God's throne; nor by the earth, for it is His footstool; neither by Jerusalem, for it is the city of the great King.” For these words are evidently spoken with reference to the Creator, as also Esaias says: “Heaven is my throne, the earth is my footstool.” And besides this Being there is no other God; otherwise He would not be termed by the Lord either “God” or “the great King;” for a Being who can be so described admits neither of any other being compared with nor set above Him. For he who has any superior over him, and is under the power of another, this being never can be called either “God” or “the great King.”
Source: Adversus haereses (New Advent)