During the time that the heresy of Sabellius and Paul of Samosata began to spread, St Gregory the Wonderworker (ca. 213-270 a.d.) prayed fervently and diligently imploring God and His most pure Mother to reveal to him the true Faith. The All-Holy Virgin Mary appeared to him, radiant like the sun, and with Her was the Apostle John the Theologian dressed in archepiscopal vestments.
By the command of the Mother of God, the Apostle John taught the saint how to correctly and properly confess the Mystery of the Most Holy Trinity. St Gregory wrote down everything that St John the Theologian revealed to him. The Mystery of the Symbol of the Faith, written down by St Gregory of Neocaesarea, is a great divine revelation in the history of the Church. The teaching about the Holy Trinity in Orthodox Theology is based on it. Subsequently it was used by the holy Fathers of the Church: Basil the Great, Gregory the Theologian, and Gregory of Nyssa. The Symbol of St Gregory of Neocaesarea was later examined and affirmed in the year 325 by the First Ecumenical Council, showing his enduring significance for Orthodoxy. Thus, the first recorded vision of the Theotokos in the history of the Church was for the purposes of revealing a trinitarian confession of faith. St. Gregory of Nyssa wrote St. Gregory’s biography and he records this magnificient event:
St. Gregory of Nyssa ca. 335-395
While [Gregory] was passing a sleepless night because of these worries, someone appeared to him in human form, aged in appearance, clothed in garments denoting a sacred dignity, with a face characterized by a sense of grace and virtue. Gregory, looking frightened, rose from his bed and asked him who he was and why he had come.
The other, in a subdued voice, after soothing his distress, told Gregory that he had appeared by divine will, because of the questions that Gregory found ambiguous and confusing, to reveal to him the truth of pious faith. After hearing these words, Gregory regained his serenity and began to observe the other man with a certain joy and wonder.
The other then held up his hand, as if to point out, with his index finger, something that had appeared opposite him. Gregory, turning his gaze in the direction indicated by the other man’s hand, saw before him another figure, which had appeared not long before. This figure had the appearance of a woman, whose noble aspect far surpassed normal human beauty. Gregory was again disturbed. Turning away his face, he averted his glance and was filled with perplexity; nor did he know what to think of this apparition, which he could not bear to look upon with his eyes. For the extraordinary character of the vision lay in this: that though it was a dark night, a light was shining, and so was the figure that had appeared to him, as if a burning lamp had been kindled there.
Although he could not bear to look upon the apparition, Gregory heard the speech of those who had appeared, as they discussed the problems that were troubling him. From their words, Gregory not only obtained an exact understanding of the doctrine of the faith but also was able to discover the names of the two persons who had appeared to him, for they called each other by name.
For it is said that he heard the one who had appeared in womanly form exhorting John the Evangelist to explain to the young man the mystery of the true [faith]. John, in his turn, declared that he was completely willing to please the Mother of the Lord even in this matter and that this was the one thing closest to his heart. And so the discussion coming to a close, and after they had made it quite clear and precise for him, the two disappeared from his sight. (Life of St. Gregory the Wonderworker, PG 46, 909-12)
St. Gregory’s Confession of Faith
There is one God, the Father of the living Word, who is His subsistent Wisdom and Power and Eternal Image: perfect Begetter of the perfect Begotten, Father of the only-begotten Son.
There is one Lord, Only of the Only, God of God, Image and Likeness of Deity, Efficient Word, Wisdom comprehensive of the constitution of all things, and Power formative of the whole creation, true Son of true Father, Invisible of Invisible, and Incorruptible of Incorruptible, and Immortal of Immortal and Eternal of Eternal.
And there is One Holy Spirit, having His subsistence from God, and being made manifest by the Son, to wit to men: Image of the Son, Perfect Image of the Perfect; Life, the Cause of the living; Holy Fount; Sanctity, the Supplier, or Leader, of Sanctification; in whom is manifested God the Father, who is above all and in all, and God the Son, who is through all.
There is a perfect Trinity, in glory and eternity and sovereignty, neither divided nor estranged. Wherefore there is nothing either created or in servitude in the Trinity; nor anything superinduced, as if at some former period it was non-existent, and at some later period it was introduced. And thus neither was the Son ever wanting to the Father, nor the Spirit to the Son; but without variation and without change, the same Trinity abideth ever.
Source for the Confession of Faith: http://www.voskrese.info/spl/thaumcreed.html