Names And The Trinity

Trinitas appellata quod fiat totum unum ex quibusdam tribus, quasi Triunitas; ut memoria, intellegentia et voluntas, in quibus mens habet in se quandam imaginem divinae Trinitatis. Nam dum tria sint, unum sunt, quia et singula in se manent et omnia in omnibus. Pater igitur et Filius et Spiritus sanctus trinitas et unitas. Idem enim unum, idem tria. In natura unum, in personis tria. Unum propter maiestatis communionem, tria propter personarum proprietatem. Nam alius Pater, alius Filius, alius Spiritus sanctus: sed alius quidem non aliud, quia pariter simplex pariterque incommutabile bonum et coaeternum. Pater solus non est de alio; ideo solus appellatur ingenitus. Filius solus de Patre est natus; ideo solus dicitur genitus. Spiritus sanctus solus de Patre et Filio procedit; ideo solus amborum nuncupatur spiritus. In hac Trinitate alia appellativa nomina, alia propria sunt. Propria sunt essentialia, ut Deus, Dominus, Omnipotens, Inmutabilis, Inmortalis. Et inde propria, quia ipsam substantiam significant qua unum sunt. Appellativa vero Pater et Filius et Spiritus sanctus, Ingenitus et Genitus et Procedens. Eadem et relativa, quia ad se invicem referuntur. Cum enim dicitur Deus, essentia est, quia ad se ipsum dicitur. Cum vero dicitur Pater et Filius et Spiritus sanctus, relative dicuntur, quia ad se invicem referuntur. Nam Pater non ad se ipsum, sed ad Filium relative dicitur, quia est ei filius: sic et Filius relative dicitur, quia est ei pater: sic et Spiritus sanctus, quia est Patris Filiique spiritus. His enim appellationibus hoc significatur, quod ad se invicem referuntur, non ipsa substantia qua unum sunt. Proinde Trinitas in relativis personarum nominibus est; deitas non triplicatur, sed in singularitate est; quia si triplicatur, deorum inducimus pluralitatem. Nomen autem deorum in angelis et sanctis hominibus ideo pluraliter dicitur, propter quod non sint merito aequales.  De quibus Psalmus: Ego dixi: Dii estis. De Patre autem et Filio et Spiritu sancto propter unam et aequalem divinitatem non nomen deorum, sed Dei esse ostenditur, sicut ait Apostolus: Nobis tamen unus Deus, vel sicut voce divina dicitur: Audi Israel, Dominus Deus tuus Deus unus est, scilicet ut et Trinitas sit, et unus Dominus Deus sit. Sanctus Isidorus Hispalensis, Etymologiae, Lib VII, Cap IV Source: Migne PL 82.271

The Trinity is so named because from a certain three is made one whole, as a 'Tri-unity', like memory, intelligence, and will, in which the mind has in itself a certain image of the Divine Trinity. For while they are three, they are one, because while being in themselves as individuals, they are all in all. Therefore the Father and the Son and the Holy Spirit are a Trinity and a unity, for they are one and they are three. In nature one, in person three. One because of shared majesty, three because of the persons. For the Father is one person, the Son another, the Holy Spirit another, but another person, not another thing, because they are equally a single thing and equally immutable, good, and coeternal. The Father alone is not from another; therefore He is named Unbegotten. The Son alone is born of the Father; therefore He is named Begotten. The Holy Spirit alone proceeds from the Father and the Son; therefore is alone called the Spirit of both. In this Trinity some names are appellative, and some are proper. The proper ones refer to the essence, such as God, Lord, Almighty, Immutable, Immortal. And they are proper because they signify the very substance by which they are one. However appellative names are Father and Son and Holy Spirit, Unbegotten and Begotten and Proceeding. And these are relational because they relate to one another. When one says 'God,' it is the essence, because He is being named with respect to Himself. But when one says Father and Son and Holy Spirit, these names are spoken relationally, because they relate to one another. For 'Father' is not with respect to Himself, but with respect to His relation to the Son, because He has a Son; and so 'Son' is spoken relationally, because He has a Father; and so with 'Holy Spirit,' because it is the Spirit of the Father and the Son. By these appellatives is signified that by which they relate to one another, but not the substance itself in which they are one. Hence the Trinity exists in the relational names of the persons. Deity is not tripled, but is in singleness, for if it were tripled we would introduce a plurality of gods. For that reason the name of gods in the plural is spoken of about angels and holy men, because they are not His equal in merit. Concerning which the Psalm says, 'I have said: You are gods.' 1 But for the Father and Son and Holy Spirit, because of their one and equal Divinity, the name is shown to be not gods but God, as the Apostle says: 'But for us there is one God,' 2 or as it is said by the Divine voice, 'Hear, O Israel, the Lord your God is one God,' 3 as He is both the Trinity and the one Lord God. Saint Isidore of Seville, from The Etymologies, Book 7, Chap 4 1 Ps 81.6 2 1 Cor 8.6 3 Mark 12.29, Deut 6.4