Tertullian, Against Marcion 3
So the whole debate comes down to numbers: whether we're allowed to posit two gods — maybe by poetic license, maybe by artistic license, and now, heretical license too. But Christian truth calls it straight: if God isn't one, God isn't. Period. Because we hold it more fitting that nothing exists at all than that something exists in a way it shouldn't.
Want to know why God has to be one? Ask what God is. You won't find any other answer. As far as human language can stretch, here's the definition — and every conscience will back me up: God is the supreme greatness, fixed in eternity, unborn, unending. That's what eternity means: the very condition that makes God supremely great, because that condition *is* God. And from that flows everything else — God is supreme in form, in reason, in power.
Everyone agrees on this — no one denies God is the supreme greatness, unless they're ready to call God the lowest trifle instead, which is just denying God by stripping away what makes God God. So what defines the supreme greatness? Simple: nothing equals it. Which means there *can't* be another supreme greatness. Because if there were, they'd be equals. And if they're equals, neither is supreme. The whole definition collapses — the law, if you will, that lets nothing stand alongside the supreme.
So the supreme greatness *must* be unique — no equal, or it stops being supreme. It exists *as* unique; that's the only way it exists at all. And since God *is* the supreme greatness, our truth got it right: if God isn't one, God isn't. Not that we're hedging — 'maybe God doesn't exist if not one.' No. We're defining the God we trust: this is what God *is*. If that's not true, God isn't.
So if the supreme greatness has to be unique, God has to be unique. God isn't God unless supreme; supreme isn't supreme unless peerless; peerless isn't peerless unless unique. Try smuggling in another god — you'll have to give him every divine attribute: eternal, supreme, the works. Then you've got two supreme greatnesses. How does that work? The definition of supreme is *having no peer*. That's a singular property. Two things can't both have no peer. They'd be each other's peer. Contradiction. Game over.
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?