One God – Yes:
One God, One Person – No:
One God, Three Persons – Yes:
Three Persons, One God – Yes;
A Trinity in Unity and a Unity in Trinity – Yes!
We beseech thee, that through the steadfastness of this faith, we may evermore be defended against all adversity.
Glory be to the Father, and to the Son, and to the Holy Ghost, now and for always, even unto ages of ages .
The Church in the West was very wise, and no doubt led by the Holy Ghost, to call the Sunday after Whitsuntide (Pentecost) by the name of Trinity Sunday, in order that the focus of worship and devotion be most particularly on that day the Triune LORD God himself—the Father, the Son and the Holy Ghost, Three Persons One God, a Trinity in Unity and a Unity in Trinity.
The major festivals of the Christian Year before Trinity Sunday focus on (a) the Incarnation of the Second Person of the Trinity, his taking of our human nature and flesh as his own [Christmas & Epiphany]; (b) the sacrificial, atoning death of the Second Person for our sins and his rising again from the dead for our justification [Easter]; (c) the ascending into heaven with his assumed and now glorified human nature of the Second Person to be the High Priest and King of his people [Ascension]; and (d) his sending, together with the Father, of the Holy Ghost to the Church in order for the Third Person of the Trinity to be the Paraclete of the Incarnate Son, a Counselor and Comforter to his sanctified people [Whitsuntide or Pentecost].
In the great work of divine revelation and redemption, salvation and sanctification, the Holy Trinity is sovereignly, supremely and wholly involved, as the Father sends the Son into the world where he assumed human nature by the presence of the Holy Ghost, and where the Holy Ghost acts in the Name of the Son. So it is most fitting and most appropriate that after the sequence of the great festivals – Christmas & Epiphany, Easter, Ascension and Whitsuntide – there should be another festival pointing to the identity of the One LORD our God, the God of creation, revelation and redemption, by whom the divine reality of the great festivals is assured.
Jesus himself commanded that disciples be baptized "In the Name of the Father, and of the Son, and of the Holy Ghost," and the Early Church gave a lot of time and effort to the stating in the best possible and available terms the doctrine of the Holy, Blessed and Undivided Trinity of the Father, the Son and of the Holy Ghost. That is, the work of rendering of the dynamic biblical teaching and insights concerning the LORD (YHWH) as Triune into clear propositional terms, using particular words in specific ways. This teaching is found in the Nicene Creed (written originally in Greek and immediately translated into Latin) and in the Athanasian Creed or Quicunque Vult (written originally in Latin and later translated into Greek).
Key words are substance (ousia in Greek) and person (hypostasis in Greek). And the church teaching is that there is one ousia (Divinity, Deity, Godhead, Substance) and that each of the Three Persons possesses in whole this one, unique ousia. This one substance or essence that is God (Godhead), is not, as it were, equally shared and split into three. The Father is wholly God; the Son is wholly God and the Holy Ghost is wholly God. Thus the Father and the Son and the Holy Ghost are homoousios (of the same, identical substance, essence & being) with each other.
The Three Persons differ from one another not in Godhead for each one is wholly God; rather they differ in terms of their relations one with another. The first Person is the Father of the Only-Begotten Son; the Son is the only-begotten Son of the Father; and the Holy Ghost proceeds from the Father and the Son. And, of course, in the divine work of creation, redemption, providence and judgment, each of the Three has a different but not an independent role.
It is this Mystery, God as the Holy Trinity, whom holy Mother Church asks all her members ( born from above by the Holy Ghost in the Name of the Son to be the adopted children of the Father) specifically to adore, praise and magnify on Trinity Sunday, and to do so with special effort, concentration and devotion.
In the late medieval Church this act of praise was offered om Trinity Sunday after the recital of the Athanasian Creed:
Blessed and glorious Trinity, Father, Son and Holy Spirit, thanks be to Thee, very and one Trinity, one and perfect Godhead. Thee, God the Father Unbegotten; Thee, the Only-begotten Son; Thee, the Holy Spirit, the Paraclete; the Only and Undivided Trinity, do we confess and praise with heart and mouth; to Thee be glory for ever. Alleluia.
Then for the rest of the [traditional Western] Christian Year until Advent, as each Sunday in the traditional Anglican Calendar also bears the Name of the same Holy Trinity, holy Mother Church asks her members to hear and read the Gospel and the Epistle as the words of the same Triune God, even as she worships the Undivided and Blessed Trinity, bowing before the Father in the Name of the Son and with the presence and illumination of the Holy Ghost. It is only when we know God as the Triune Lord God experientially and mentally that we are aware of the need for careful terminology both to preserve sound doctrine and to honor God for who he is (God-as-he-is-unto-himself) and how and what he has revealed of himself unto us (God-as-he-is-towards-us).
In 1549 the traditional Latin Collect for Trinity Sunday was rendered into English by Archbishop Cranmer in this form:
Almighty and everlasting God, who hast given unto us thy servants grace, by the confession of the true faith, to acknowledge the glory of the eternal Trinity, and in the power of the divine Majesty to worship the Unity: We beseech thee, that through the steadfastness of this faith, we may evermore be defended against all adversity, who livest and reignest, one God, world without end. Amen .
(In 1662 this was modified to become: Almighty and everlasting God, who hast given unto us thy servants grace, by the confession of the true faith, to acknowledge the glory of the eternal Trinity, and in the power of the divine Majesty to worship the Unity: We beseech thee, that thou wouldest keep us steadfast in this faith, and evermore defend us from all adversities, who livest and reignest, one God, world without end. Amen.)
Let us explore the meaning of the petition in the original form of the Collect – which asks that by steadfastness in this Faith (in the One, Holy, Blessed and Undivided Trinity) we are to be safe from evil. In other words, that the authentic Christian Creed is to be the shield of the life of each and every true believer.
In the Book of Proverbs we read: "The Name of the LORD is a strong tower; the righteous man runs into it and is safe" (18:10).
Baptized into Christ Jesus in the one Name of the Father, the Son and the Holy Ghost, confessing this one Name in the Apostles' , Nicene and Athanasian Creeds, and being blessed in church by this same Name, the baptized children of God worship the Father through the Incarnate Son and by the Holy Ghost. Thus they are, as it were, enclosed by the Name; they inhabit the Name, and they are protected by the Name against the wiles and attacks of all spiritual and physical foes. And. In biblical terms, to be protected by the Name is to be protected by the TRINITY, by the Father through the Son and with the Holy Ghost.
The Holy Trinity is NOT and has NEVER been a doctrine (although there is a doctrine of the Trinity). Rather it is the very Name of the LORD God, who is a Trinity in Unity and a Unity in Trinity.
(See further Peter Toon, Our Triune God, Regent College Publishing, Vancouver, Canada)
P.S. Perhaps the greatest error amongst many errors in doctrine in the 1979 Prayer Book of The Episcopal Church is in the Catechism (itself based only the Rite 2 material of the same Book) at page 852. In the short section on the Creeds five questions beginning with "What?" (e.g., "What is the Apostles' Creed?") appear, and the last of these is "What is the Trinity?" In the Christian Tradition, the TRINITY is not a "what" but the unique "Who"—the one, eternal, undivided Godhead (ousia, substance/essence), wholly possessed by each of the Three Persons, A Unity in Trinity and a Trinity in Unity.