Tertullian, Against Marcion 3
The whole debate centers on the number: whether two gods can be posited — perhaps by poetic and pictorial license, and now also by heretical license. But Christian truth has spoken distinctly: if God is not one, he is not God, because we believe it is more worthy for whatever is not as it ought to be not to exist at all. So that you may know God must be one, seek what God is, and you will find no other answer. As far as the human condition can define God, I define this, which every conscience will also acknowledge: God is the Highest, the Great, established in eternity — innate, unmade, without beginning, without end — for this state is what constitutes eternity, which makes God the Highest Great God while this state exists in God himself; and thus also the rest, so that God is the Highest Great One in form and reason and power. Since everyone agrees on this — for no one will deny that God is the Highest Great One, except someone who could pronounce the opposite, that God is the lowest small one, thereby denying God by taking away what belongs to God — what will be the condition of this Highest Great One? Namely, that nothing equals it; that is, that there is no other Highest Great One, because if there were, it would be equal, and if it were equal, it would no longer be the Highest Great One, with the condition overturned and, so to speak, the law that allows nothing to equal the Highest Great One. Therefore the Highest Great One must be unique — not having a peer — so that it may remain the Highest Great One. Therefore it cannot be otherwise than through what it has its being by, that is, unique altogether. Therefore, since God is the Highest Great One, our truth has pronounced rightly: God, if not one, is not God — not as if we doubt that God exists by saying 'if not one, he is not,' but because we define as God the very One we confess to be God, so that if that One is not, God is not. Furthermore, if the Highest Great One must be unique, then God will also be unique — not God in any other way except as the Highest Great One, and not the Highest Great One in any other way except by not having a peer, and not not having a peer in any other way except by being unique. Certainly, whatever other god you introduce, you will not be able to preserve him as God in any other form than by ascribing to him also the property of divinity, just as eternal, so also the Highest Great One. How, then, can two Highest Great Ones coexist, since to be the Highest Great One is not to have a peer, and not having a peer belongs to one alone, and in two it cannot exist in any way?
Latin
Principalis itaque et exinde tota congressio de numero, an duos deos liceat induci, si forte, poetica et pictoria licentia, et tertia iam, haeretica. sed ueritas Christiana destricte pronuntiauit: deus si non unus est, non est, quia dignius credimus non esse quodcumque non ita fuerit, ut esse debebit. deum autem ut scias unum esse debere, quaere quid sit deus, et non aliter inuenies. quantum humana condicio de deo definire potest, id definio, quod et omnium conscientia agnoscet: deum summum esse magnum in aeternitate constitutum, [innatum, infectum, sine initio, sine-fine] — hunc enim statum aeternitati t censendum, quae summum magnum deum efficiat, dum hoc est in deo ipsa — atque ita et cetera, ut sit deus summum magnum et forma et ratione et ui et potestate. cum de isto conuenit apud omnes, — nemo enim negabit deum summum magnum quid esse, nisi qui potuerit deum imum modicum quid e contrario pronuntiare, ut deum neget auferendo quod dei est - quae erit iam condicio ipsius summi magni? nempe, ut nihil illi adaequetur, id est, ut non sit aliud summum magnum, quia, si fuerit, adaequabitur, et si adaequabitur, non erit iam summum magnum, euersa condicione et, ut ita dixerim, lege, quae summo magno nihil sinit adaequari. ergo unicum sit necesse est summum magnum, — quod fuerit par non habendo — ne non sit summum magnum. ergo non aliter erit quam per quod habet esse, id est unicum omnino. proinde deus cum summum magnum sit, recte ueritas nostra pronuntiauit: deus, si non. unus est, non est, non quasi dubitemus esse deum dicendo: si non unus, non est, sed quia quem confidimus esse, id eum definiamus esse, quod si non est, deus non est. [et summum scilicet magnum] porro (si) summum magnum unicum sit necesse est, ergo et deus unicus erit, non aliter deus, nisi summum magnum, nec aliter summum magnum, nisi parem non habens, nec aliter parem non habens, nisi unicus fuerit. certe quemcumque alium deum induxeris, non alia poteris eum forma tueri deum quam ut et illi proprium diuinitatis adscripseris, sicut aeternum ita et summum magnum. duo ergo summa magna quomodo consistent, cum hoc sit summum magnum, par non habere, par autem non habere uni competat, in duobus esse nullo modo possit?