Tertullian, Against Marcion 1.3
Therefore the principal and thereafter the whole contest is about number: whether it is allowed to bring in two gods—by poetic and pictorial license if you like, and now also by a third, heretical. But Christian truth has pronounced stripped bare: if God is not one, he is not, because we more worthily believe that whatever is not as it ought to be does not exist. But that you may know God ought to be one, ask what God is, and you will not find otherwise. As far as human condition can define about God, I define what even everyone’s conscience will acknowledge: that God is the supreme great established in eternity—unborn, unmade, without beginning, without end—for this status is to be counted for eternity, which makes God the supreme great while this itself is in God—and so also the rest, that God may be the supreme great in form and reason and force and power.
When all agree on this—for no one will deny that God is some supreme great unless he could pronounce God some lowest little thing by contrary, so as to deny God by taking away what is God’s—what will now be the condition of the supreme great itself? Surely, that nothing be equaled to it—that is, that there be no other supreme great—because if there were, it would be equaled, and if equaled it would no longer be the supreme great, the condition and, so to speak, the law overturned which allows nothing to be equaled to the supreme great. Therefore the supreme great must necessarily be unique—which will have been so by having no peer—lest it not be the supreme great. Therefore it will not otherwise be than through that by which it has to be—that is, altogether unique.
Accordingly, since God is the supreme great, our truth rightly pronounced: God, if he is not one, is not—not as if we doubt that God exists by saying ‘if not one, he is not,’ but because him whom we trust exists we define to be that which if he is not, God is not. Further, if the supreme great must necessarily be unique, therefore God also will be unique—not otherwise God unless the supreme great, nor otherwise the supreme great unless having no peer, nor otherwise having no peer unless he has been unique. Certainly whatever other god you bring in, you will not be able to guard him as god by any other form than by ascribing to him also what is proper to divinity—as eternal, so also supreme great. How then will two supreme greats stand, when this is the supreme great—to have no peer—and having no peer belongs to one, and can in no way be in two?
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?