Commentaria in Scripturam Sacram, Vol. 11: Prolegomena (prefaces and canons)
For the Father is one, the Son another, the Holy Spirit another; yet not another thing. And conversely, these three are one; yet not one person. For each person by himself is one, and therefore one person: but the one, or unity of essence, is whole in the three persons, and in each. Long since exploded, and of its own accord shattered upon itself, is the error of Abelard, who posited degrees of persons in the Most Holy Trinity, against whom St. Bernard says sublimely: “He, says he, rightly according to his ability esteems the divine magnificence, who thinks nothing unequal in it, where all is supreme; nothing distant, where all is one; nothing gaping, where all is entire: nothing imperfect or wanting, where all is whole. For the Father is whole what the Son and the Holy Spirit are. The Son is whole what he himself and the Father and the Holy Spirit are. The Holy Spirit is whole what he himself and the Father and the Son are. And the whole is one whole, neither superabundant in the three, nor diminished in each: for they do not divide among themselves, in parts, the true and supreme good which they are; since they do not possess it by participation, but are this very thing essentially,” Epist. 19 to Pope Innocent. Wondering at and deeply venerating this, St. Augustine says: “To you alone, O Trinity, you are fully known; holy Trinity, Trinity supermarvelous and superineffable, superinaccessible, superincomprehensible, superintelligible, superessential, superessentially surpassing every sense, every reason, every intellect, every essence of the supercelestial spirits: whom neither to speak of, nor to think, nor to understand, nor to know is possible, even to the eyes of the Angels,” in the Soliloquies, ch. xxxi.
Furthermore, although the works of the Most Holy Triad, like its nature, power, mind and will, are undivided, yet on account of the affinity proper to each person, omnipotence is rightly attributed to the Father, says St. Thomas, I part, Q. xlv, art. 6; omniscience to the Son, because he is the Word and the idea of the Father; supreme and immense goodness to the Holy Spirit, because he is the notional love of the Father and the Son. Again: “In the Father,” says St. Augustine, “is unity, in the Son equality, in the Holy Spirit the harmony of unity and equality. And all these three are one on account of the Father, all equal on account of the Son, all connected on account of the Holy Spirit,” bk. I On Christian Doctrine, ch. v. Moreover, eternity is ascribed to the Father, truth to the Son, charity to the Holy Spirit; to the Father providence, to the Son wisdom, to the Holy Spirit order and union; to the Father creation, to the Son government, to the Holy Spirit conservation; to the Father predestination, to the Son redemption, to the Holy Spirit sanctification; to the Father calling, to the Son justification, to the Holy Spirit glorification; to the Father purgation, to the Son illumination, to the Holy Spirit perfection; to the Father the past, to the Son the present, to the Holy Spirit the future; to the Father the beginning, to the Son the middle, to the Holy Spirit the end and term. For, as St. Bernard says, serm. 80 on the Canticle: “The Most Holy Trinity contains all places, and orders each in its own place, itself contained by no places; times pass beneath it, not it; it does not await the future, nor recall the past, nor experience the present.” For the Most Holy Trinity is immensity, to which every place, highest, middle, lowest, is present; and eternity, to which every time is present and coexists, and consequently the past is for it not past but present, the future not future but present. Wherefore the perfect knowledge of the Trinity is full wisdom and eternal life; sighing after which, St. Gregory Nazianzen thus sings in his hymn to God:
O living Trinity! I will sing you, the one monarch, to whom there is no origin, nor change, nor …
The same, orat. 40, calls the Most Holy Trinity “the infinite conjunction of three infinites. I have scarcely conceived one,” he says, “in my mind, when I am encircled with the splendor of three. I scarcely begin to distinguish the three, when at once I am borne aloft to the one, etc. I behold one splendor, as one who cannot distinguish or divide the united light.” For, as the same says in his poem on the Holy Spirit, in the Most Holy Triad there is a monad, and therefore a monarchy, which makes concord; not a polyarchy, which breeds discord, strife and wars. Therefore with whole heart and mouth let us celebrate the triad, adore the monad, bless the Father, the Son and the Holy Spirit, let us praise and exalt him above all for ever.
Now the Most Holy Trinity has communicated itself to Christ and to us in many ways, and has wrought a certain trinity, and works and communicates it daily. “There are three,” says St. John, “who give testimony in heaven: the Father, the Word, and the Holy Ghost: and these three are one. And there are three that give testimony on earth: the spirit, and the water, and the blood.”
St. Bernard: “As in that singular divinity there is a trinity in persons, a unity in substance: so in this special commingling (in Christ) there is a trinity in substances, a unity in person. And as there the persons do not sunder the unity, nor does the unity lessen the trinity: so here too the person does not confound the substances, nor do the substances themselves dissipate the unity of the person. That supreme Trinity has shown us this trinity, a work, a marvelous work, singular among all and above all its works. For the Word, and soul, and flesh came together into one person; and these three are one, and this one is three, not by confusion of substance, but by unity of person,” serm. 3 on the Vigil of the Nativity. Blessed, therefore, be the holy Trinity and undivided Unity; we will give praise to him, because in Christ he has done his great mercy with us.