How can we understand the oneness of the Holy Trinity?

The oneness of the Holy Trinity is unique. It is not a mixture, as happens with liquids or materials; nor is it like the unity of the human body and soul; nor is it like the union of the divinity of Christ with His humanity. Each of the three Hypostases fills the other two and is contained within them, and yet remains distinct from them.

St. Athanasius says, “And the Son, being the proper Offspring of the Father’s Essence, reasonably says that the Father’s attributes are His own also; whence suitably and consistently with saying, ‘I and the Father are One,’ (John 10:30) He adds, ‘that you may know that I am in the Father and the Father in Me’ (John 14:10). Moreover, He has added this again, ‘He that has seen Me, has seen the Father’ (John 14:9); and there is one and the same sense in these three passages. For he who in this sense understands that the Son and the Father are one, knows that He is in the Father and the Father in the Son; for the Godhead of the Son is the Father’s, and it is in the Son … the fulness of the Father’s Godhead is the Being of the Son, and the Son is whole God … the Godhead and the Form of the Son is none other’s than the Father’s, this is what He says, ‘I in the Father.’ Thus ‘God was in Christ reconciling the world unto Himself’ (2 Cor. 5:19); for the propriety of the Father’s Essence is that Son, in whom the creation was then reconciled with God.”³⁷¹

He also says, “For the holy and blessed Triad is indivisible and one in itself. When mention is made of the Father, there is included also his Word, and the Spirit who is in the Son . If the Son is named, the Father is in the Son, and the Spirit is not outside the Word. For there is from the Father one grace which is fulfilled through the Son in the Holy Spirit.”³⁷²

St. Hilary of Poitiers says of the oneness of the Holy Trinity: “He commanded them to baptize in the name of the Father, the Son, and the Holy Spirit, that is, in the confession of

370 St. Athanasius of Alexandria, Four Discourses Against the Arians 1:28 (NPNF II/4:322) amended. See also Defence of the Nicene Definition 17 (NPNF II/4:160–161).

371 St. Athanasius of Alexandria, Four Discourses Against the Arians 3.23.5 (NPNF II/4:395–396).

372 St. Athanasius of Alexandria, The Letters of Saint Athanasius Concerning the Holy Spirit to Bishop Serapion, trans. C. R. B. Shapland, (Epworth Press, 1951), Epistle 1:14: pp. 93–94.

the Origin, the Only-begotten, and the Gift. There is one source of all. God the Father is one from whom are all things; and our Lord Jesus Christ is one through whom are all things [1 Cor. 8:6]; and the Holy Spirit is one, the gift in all things [Eph. 4:4]. Everything is arranged, therefore, according to its power and merits. There is one Power from whom are all things, one Offspring through whom are all things, and one Gift of perfect hope. Nor will anything be found wanting to a perfection so great within which there is found in the Father, the Son, and the Holy Spirit: infinity in the Eternal, the form in the Image, and the use in the Gift.[”373]