Besides, if, from that perfect knowledge, and to the Holy Ghost, we were to invoke a plurality of gods and lords, we should quench our torches, and we should become less courageous to endure the martyr’s sufferings, from which an easy escape would everywhere lie open to us, as soon as we swore by a plurality of gods and lords, as sundry heretics do, who hold more gods than One. 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. – Ad. Prax. XIIIwhich assures us that the title of God and Lord is suitable both to the Father, and to the Son
Tertullian’s formula was: three personaes, one substantia.
It is interesting to read Tertullain, especially agains the backdrop of Justin who readily said two Gods, Father and Son. Tertullain, no doubt battling not just the Sabellianism which denied the distinction of the Father and the Son (the Son being a nickname) but also the plurality of the Godhead. In the chapter mentioned above, Tertullian rails against those that promote two Gods – Father and Son. In the end, Tertullian describes the Son as God in the way that a sunbeam is the Sun. There is no distinction in the Substance of the Sun and Sunbeam, only in the modes of appearance.
It is interesting that Tertullian retreats to the formula of the Apostles.