Very well! you say, I shall challenge you to preach from this day forth (and that, too, on the authority of these same Scriptures) two Gods and two Lords, consistently with your views. God forbid, (is my reply). For we, who by the grace of God possess an insight into both the times and the occasions of the Sacred Writings, especially we who are followers of the Paraclete, not of human teachers, do indeed definitively declare that Two Beings are God, the Father and the Son, and, with the addition of the Holy Spirit, even Three, according to the principle of the divine economy, which introduces number, in order that the Father may not, as you perversely infer, be Himself believed to have been born and to have suffered, which it is not lawful to believe, forasmuch as it has not been so handed down. That there are, however, two Gods or two Lords, is a statement which at no time proceeds out of our mouth: not as if it were untrue that the Father is God, and the Son is God, and the Holy Ghost is God, and each is God; but because in earlier times Two were actually spoken of as God, and two as Lord, that when Christ should come He might be both acknowledged as God and designated as Lord, being the Son of Him who is both God and Lord. Now, if there were found in the Scriptures but one Personality of Him who is God and Lord, Christ would justly enough be inadmissible to the title of God and Lord: for (in the Scriptures) there was declared to be none other than One God and One Lord, and it must have followed that the Father should Himself seem to have come down (to earth), inasmuch as only One God and One Lord was ever read of (in the Scriptures), and His entire Economy would be involved in obscurity, which has been planned and arranged with so clear a foresight in His providential dispensation as matter for our faith. As soon, however, as Christ came, and was recognised by us as the very Being who had from the beginning caused plurality (in the Divine Economy), being the second from the Father, and with the Spirit the third, and Himself declaring and manifesting the Father more fully (than He had ever been before), the title of Him who is God and Lord was at once restored to the Unity (of the Divine Nature), even because the Gentiles would have to pass from the multitude of their idols to the One Only God, in order that a difference might be distinctly settled between the worshippers of One God and the votaries of polytheism.
…I will therefore not speak of gods at all, nor of lords, but I shall follow the apostle; so that if the Father and the Son, are alike to be invoked, I shall call the Father “God,” and invoke Jesus Christ as “Lord.” But when Christ alone (is mentioned), I shall be able to call Him “God,” as the same apostle says: “Of whom is Christ, who is over all, God blessed for ever.” For I should give the name of “sun” even to a sunbeam, considered in itself; but if I were mentioning the sun from which the ray emanates, I certainly should at once withdraw the name of sun from the mere beam. For although I make not two suns, still I shall reckon both the sun and its ray to be as much two things and two forms of one undivided substance, as God and His Word, as the Father and the Son.
Tertullian
Against Praxeas
Chapter XIII