Summa de Ecclesia

After discourse has been had of the quality and mode of the union by which the members of the body of the Church are united and connected to one another, and of its causes, it is consequent that we touch upon something of the quality and mode of the union by which the members themselves are united to Christ their head. For which it is to be noted, as Hugh of St. Victor says in his book on the sacraments, part 2, treating that word of John 17, Father, keep those whom thou hast given me, that they may be one as we also are one. A threefold unity is spoken of¹, namely unity in nature, and in this way the Father and the Son and the Holy Spirit are one. John 10: I and the Father are one. And 1 John 5: There are three who bear witness in heaven, the Father, the Word, and the Holy Spirit, and these three are one through an undivided, namely the same, nature. For as Aug. says in book 1 against Maximinus the Arian heretic, it is one thing to say they are one, and another to say he is one. When it is said they are one, and it is not said in what they are one, unity of substance is understood. When it is said he is one, of two diverse substances, it must necessarily be asked in what he is one. For example, diverse is the substance of soul and body, yet one man; and diverse is the substance of the spirit of man and the Spirit of God, and yet when it adheres to God it is one spirit. He says one, not they are one; but where one is said, one substance is signified; thus Aug. The second unity is called personal unity, by which unity humanity and divinity are one in the person of the Son of God. Whence Athanasius in the creed: Christ is altogether one, namely, not by confusion of substance, but by unity of person. The third is unity through conformity of will, according to that word of Acts 4: They had one heart and one soul. Treating which word, Aug. on John in sermon 34 says thus: The charity of God is poured forth in our hearts by the Holy Spirit who is given to us, makes many souls one soul, and many hearts makes one heart. And in his book On True Innocence: Of many men there are without doubt many souls, and many hearts; if, however, through faith and love they adhere to God, they all become one soul and one heart. The first and most perfect unity is that of the divine persons in one nature, of which Aug. against Maximinus the heretic in book 3 speaks thus: The three persons are one, and supremely one, where there is no diversity of natures, where there is no diversity of wills. The second is the unity of natures in the one person of Christ, of which Aug. says in the same place: Moreover Christ is one person of a twofold substance, because he is God and man. The third is the unity of the head and the body of the Church, which is a unity of conformity in one will, and this is through present justice. Whence Aug. in his book On True Innocence: They who walk the ways of justice tend toward the likeness of their own author. And Jerome upon that word of Psalm 2, The innocent and the upright have adhered to me, says thus: As the neck, head, and members are contained in the same body, so also the Church adheres to Christ through justice, and her members are contained in the same body through justice.

Rightly indeed is the body of the Church said to adhere to Christ, since justice itself, according to Anselm, is a certain rectitude of will, conforms souls to God. Of which conformity the Apostle speaks at Rom. 8: Whom he foreknew, he also predestined to be made conformable to the image of his Son; the Gloss: in purity of life now, and afterwards in beatitude. Of these three unities the venerable Hugh speaks thus in the same place: The Word with the Father is one in nature, man with the Word is one in person, the members with the head are one first in justice, and afterwards one in glory. But as within the members of the human body and the head there is not only unity of spirit by which the whole body lives and is quickened, but also conformity of nature inasmuch as they are of like parts and of one nature, namely from flesh and bone, so also between the members of the mystical body and Christ the head there is union through conformity of will, of which mention was made, or through unity of spirit, of which the Apostle says at Rom. 8: He who adheres to God is one spirit with him, which unity gratuitous love itself effects, for love has, as Dionysius says in his book on the divine names, a unitive power. Whence it is said at 2 John 4: He who abides in charity abides in God, and God in him. For the sake of this the Apostle exhorts at Col. 3: Above all things have charity, which is the bond of perfection, for charity, as the Gloss says, gathers all into one. There is besides, between the members of the body of Christ and Christ the head, union through conformity of one and the same nature in species, which through the incarnation of the Son of God was granted to the Church taken from among men. Whence the Word was made flesh and dwelt among us, John 1, that is, being found like unto us, as man, as the Gloss says. Hence blessed Aug. on Psalm 148 says that the Word was made flesh that he might be head of the Church, since, according as the Gloss says upon that word at Eph. 1, He gave him to be head over all the Church: Christ is properly called head according to his humanity, according to which the Church is conjoined to him both in nature and in grace. This twofold unity, moreover, namely of nature and of grace, between Christ and the Church is openly designated in Cant. 4, where under the appellations of sister and spouse of Christ the holy Church herself is described; for it is said in Cant. 4: A garden enclosed, my sister spouse. By the name of spouse indeed the unity of affection or grace, but by the word sister the conformity of nature, are also most clearly designated.