Lactantius — Divine Institutes 4.29
Perhaps someone may ask how, when we say we worship one God, we nevertheless assert that there are two, God the Father and God the Son. This assertion drives many into the greatest error. Though what we say seems probable to them, they think we waver in this one point—that we confess both another God and a mortal God. We have already spoken about mortality; now let us teach about unity. When we say God the Father and God the Son, we do not say something different, nor do we separate the two, because neither can the Father be separated from the Son nor the Son from the Father, if indeed the Father cannot be named without the Son nor can the Son be generated without the Father. When therefore the Father makes a son and the Son a father, there is one mind for both, one spirit, one substance: but the former is like an overflowing fountain, the latter like a stream flowing down from him; the former like the sun, the latter like a ray stretched from the sun. And since he is both faithful and dear to the highest Father, he is not separated, just as neither stream from fountain nor ray from sun, because both the fountain’s water is in the stream and the sun’s light is in the ray; equally neither voice can be severed from the mouth nor virtue or hand torn away from the body.
Wherefore, when the mind and will of the one is in the other—or rather one in both—rightly either is called one God, because whatever is in the Father flows across to the Son and whatever is in the Son descends from the Father. Therefore that highest and singular God cannot be worshiped except through the Son. Whoever thinks he worships the Father alone, just as he does not worship the Son, so he does not even worship the Father. But whoever receives the Son and bears his name—that one truly worships the Father together with the Son, since the Son is legate and messenger and priest of the highest Father. This one is the door of the greatest temple, this the way of light, this the leader of salvation, this the doorway of life.
Latin
Fortasse quaerat aliquis quomodo, cum deum nos unum colere dicamus, duos tamen esse adseueremus, deum patrem et deum filium. quae adseueratio plerosque in maximum inpingit errorem. quibus cum probabilia uideantur esse quae dicimus, in hoc uno labare nos arbitrantur, quod et alterum et mortalem deum fateamur. de mortalitate iam diximus: nunc de unitate doceamus. cum dicimus deum patrem et deum filium, non diuersum dicimus nec utrumque secernimus, quia nec pater a filio potest nec filius a patre secerni, siquidem nec pater sine filio nuncupari nec filius potest sine patre generari. cum igitur et pater filium faciat et filius patrem, una utrique mens, unus spiritus, una substantia est: sed ille quasi exuberans fons est, hic tamquam defluens ex eo riuus, ille tamquam sol, hic quasi radius ex sole porrectus. qui quoniam summo patri et fidelis et carus est, non separatur, sicut nec riuus a fonte nec radius a sole, quia et aqua fontis in riuo est et solis lumen in radio; aeque nec uox ab ore seiungi nec uirtus aut manus a corpore diuelli potest. cum igitur a prophetis idem manus dei et uirtus et sermo dicatur, utique nulla discretio est, quia et lingua, sermonis ministra, et manus, in qua est uirtus, indiuiduae sunt corporis portiones. propiore uti exemplo libet. cum quis habet filium quem unice diligat, qui tamen sit in domo et in manu patris, licet ei nomen domini potestatemque concedat, ciuili tamen iure et domus una et unus dominus nominatur. sic hic mundus una dei domus est et filius ac pater, qui unanimes incolunt mundum, deus unus, quia et unus est tamquam duo et duo tamquam unus. neque id mirum, cum et filius sit in patre, quia pater diligit filium, et pater in filio, quia uoluntati patris fideliter paret, nec umquam faciat aut fecerit nisi quod pater aut uoluit aut iussit. denique unum deum esse tam patrem quam filium Esaias in illo exemplo quod superius posuimus ostendit, cum diceret: adorabunt te et in te precabuntur: quoniam in te deus est, et non est deus alius praeter te. sed et alio loco similiter ait: sic dicit deus rex Israhel et qui eruit eum deus aeternus: ego primus et ego nouissimus et praeter me non est deus. cum duas personas proposuisset, dei regis, id est Christi, et dei patris, qui eum post passionem ab inferis excitauit, sicut ostendisse diximus Osee prophetam qui ait: et de manu inferorum eruam eum, tamen ad utramque personam referens intulit: et praeter me non est deus, cum posset dicere \'praeter nos\': sed fas non erat plurali numero separationem tantae necessitudinis fieri. unus est enim, solus, liber, deus summus, carens origine, quia ipse est origo rerum et in eo simul et filius et omnia continentur. quapropter cum mens et uoluntas alterius in altero sit uel potius una in utroque, merito unus deus uterque appellatur, quia quidquid est in patre, ad filium transfluit et quidquid est m filio, a patre descendit. non potest igitur summus ille ac singularis deus nisi per filium coli. qui solum patrem se colere putat, sicut filium non colit ita ne patrem quidem. qui autem filium suscipit et nomen eius gerit, is uero cum filio simul et patrem colit, quoniam legatus et nuntius et sacerdos summi patris est filius. hic templi maximi ianua est, hic lucis uia, hic dux salutis, hic ostium uitae.