The one true God and the only Son
Epiphanius is writing to the clergy of Syedra, praising their request and explaining that knowledge of the one true God leads to unity, not polytheism.
I am truly astonished, and with good reason, at the plan of our Lord and God. In the overflowing abundance of his goodness, he was pleased to give his Holy Spirit to all who seek him in truth. My dear friends, I did not regard the requests you and those with you sent in your letters as godless. Instead, I tested the impulse behind them and found that it came from the grace of God at work in you.
For in truth, you know and agree together that those who speak in harmony with the most blessed Peter—saying, "You are the Christ, the Son of the living God"—will clearly be blessed by the Lord himself, just as that holy man was blessed when he heard, "Blessed are you, Simon son of Jonah," that is, son of Jonah. His father was called Jonah, and the word bar is translated from the Hebrew language as "son."
And this was the life that the only Son gave to his own disciples when he said that they might have life in themselves, to know the only true God and Jesus Christ whom he sent (John 17:3). So by saying "the only true God," he led us to the one rule of God, so that we would no longer be enslaved under the elements of the world, so that polytheism would no longer be in us and human thoughts would no longer be scattered toward such fornication, but toward the unity of the only true God. For Scripture says, "The invention of idols is the first fornication" (Wisdom of Solomon 14:12).
And "Jesus Christ whom he sent"—who is this Jesus Christ? He is true God. And if Christ Jesus is God, as John says about him, "The only Son, who is in the bosom of the Father, he has made him known" (John 1:18), then the Father is one God, and he alone is true God—not according to the false gods whom some of the Greeks supposed to be gods, though they are not gods. Rather, he alone is true God, because the only Son is only from the only Father, and the Holy Spirit is only. For the Trinity is in a unity, and there is one God: Father, Son, and Holy Spirit.
Greek
Καὶ σφόδρα μὲν ἐκπλήττομαι θαυμαστῶς τὴν τοῦ Κυρίου καὶ Θεοῦ ἡμῶν οἰκονομίαν, ὃς εὐδόκησε τῇ ὑπερβολῇ τῆς αὐτοῦ ἀγαθότητος ἐν ἅπασι δοῦναι τὸ ἅγιον αὐτοῦ Πνεῦμα τοῖς αὐτὸν ἐν ἀληθείᾳ ζητοῦσιν. Οὐκ ἀθέους γὰρ ὑμῶν καὶ τῶν καθ᾽ ὑμᾶς αἰτήσεις τὰς διὰ τῶν γραμμάτων ἡγησάμην, ποθεινότατοι, ἀλλὰ τὴν κίνησιν ἐδοκίμασα ἐκ τῆς τοῦ Θεοῦ χάριτος ἐν ὑμῖν ὑπάρξασαν. Ἐπ᾽ ἀληθείας γὰρ, καὶ εἰδότες συνοίδατε, καὶ σύμφωνα λέγειν τῷ μακαριωτάτῳ Πέτρῳ, ὅτι Σὺ εἶ ὁ Χριστὸς ὁ Υἱὸς τοῦ Θεοῦ τοῦ ζῶντος, σαφῶς ὑπ᾽ αὐτοῦ τοῦ Κυρίου μακαρισθήσονται, καθὼς καὶ ὁ ἅγιος ἐκεῖνος ἐμακαρίσθη ἀκούων· Μακάριος εἶ σὺ, Σίμων βὰρ Ἰωνᾶ, τουτέστιν, υἱὲ Ἰωνᾶ (ὁ γὰρ πατὴρ αὐτοῦ Ἰωνᾶς ἐκαλεῖτο· τὸ δὲ βὰρ ἑρμηνεύεται ἐκ τῆς Ἑβραϊκῆς διαλέκτου, υἱός). Καὶ τοῦτο ἦν ἡ ζωή, ἣν ἔδωκεν ὁ μονογενὴς Υἱὸς τοῖς ἑαυτοῦ μαθηταῖς λέγων· Ἵνα ἔχωσι τὴν ζωὴν ἐν ἑαυτοῖς, τὸ γινώσκειν τὸν μόνον ἀληθινὸν Θεὸν, καὶ ὃν ἀπέστειλεν Ἰησοῦν Χριστόν. Ἐν τῷ οὖν εἰπεῖν, Τὸν μόνον ἀληθινὸν Θεὸν, εἰς μοναρχίαν ἡμᾶς ἤγαγεν· ἵνα μηκέτι ὑπὸ τὰ στοιχεῖα τοῦ κόσμου ὦμεν δεδουλωμένοι, ἵνα μὴ πολυθεΐα ἐν ἡμῖν ἔτι ᾖ, καὶ ἐπὶ πορνείαν τοιαύτην ἐπισκορπίζηται τὰ τῶν ἀνθρώπων νοήματα, ἀλλ᾽ ἐπὶ ἑνότητα τοῦ μόνου ἀληθινοῦ Θεοῦ· Πρώτη γὰρ πορνεία, ἐπίνοια εἰδώλων, φησὶν ἡ Γραφή. Καὶ ὃν ἀπέστειλας Ἰησοῦν Χριστόν. Ἰησοῦν Χριστὸν τίνα; ἀληθινὸν Θεόν. Εἰ δὲ Θεὸν Χριστὸν Ἰησοῦν, ὡς λέγει περὶ αὐτοῦ ὁ Ἰωάννης· Ὁ Μονογενὴς, ὁ ὢν εἰς τὸν κόλπον τοῦ Πατρὸς, αὐτὸς ἐξηγήσατο· εἷς Θεὸς τοίνυν ὁ Πατήρ, καὶ μόνος ἀληθινὸς Θεός, καὶ οὐ κατὰ τοὺς ἐψευσμένους θεοὺς, οὓς ἐνόμισάν τινες τῶν Ἑλλήνων θεοὺς, οὐκ ὄντας θεούς· ἀλλὰ μόνος ἀληθινὸς Θεός, ἐπειδὴ μόνος ἐκ μόνου ὁ Μονογενής, καὶ μόνον τὸ ἅγιον Πνεῦμα. Τριὰς γὰρ ἐν μονάδι, καὶ εἷς Θεός, Πατὴρ, Υἱὸς καὶ ἅγιον Πνεῦμα.