Letter 29: The Trinity is known by grace and purity, not by pagan theories of spirit or fire.

I know that you have often meditated over the spiritual cup. You spoke with me about the immortality of the soul and asked me about the condition of this body: how the bodily world points the soul toward itself and, through its movements, makes known the thoughts connected with it. If you long to see the wisdom of the Creator, remember God's commandments. They purify the mind so that it becomes worthy of this spiritual cup. Wisdom will not enter a deceitful soul, nor will it dwell in a body pledged to sin. Remember the right faith. Know that the holy Trinity is not marked out in the sight of bodily things, nor in the contemplation of bodiless things, nor counted among created realities. By grace it bends down toward the soul's knowledge. Created things came from nothing, but the knowledge of the holy Trinity is substantial, unsearchable, and incomprehensible. It is not useful to look to the sages of the nations, who call God a comprehensive knowing spirit or fire, as though he had no form and changed himself into whatever he wished. That does not belong to God. It belongs to demons, whom those sages worship beyond measure, and who easily transform themselves among all bodily things.

Greek retroversion from Syriac transmission (Frankenberg 1912, TAN/TEI CC BY 4.0): γιγνωσχω σε πολλακις μελετησαι εν τωι πνευματικωι ποτηριωι· ελαλησας γαρ μετ' εμου περι της ψυχης αθανασιας και ηρωτησας με περι της καταστασεως τουτου του σωματος τον σωματικον κοσμον τηι ψυχηι αποδεικνυοντος δια δε των αυτου κινησεων τα περι αυτου νοηματα δηλουντος. ει τοινυν επιποθεις την του κτιστου σοφιαν οραν μνημονευε τας θεου εντολας τον νουν ωστε τουτου του ποτηριου πνευματικου αξιον ειναι καθαριζουσας, οτι εις κακοτεχνον ψυχην ουχ εισελευσεται σοφια ουδε κατοικησει εν σωματι καταχρεωι αμαρτιας μεμνησο της ορθης πιστεως και γνωθι οτι η αγια τριας ουχ εν οψει των σωματικων ουδε εν θεωριαι των ασωματων σημειουται ουδε τοις πραγμασιν εναριθμειται διοτι ως δια χαριτος προς την γνωσιν της ψυχης κατακκλινεται· τα γαρ κτισματα εξ ουδενος εγενετο η δε αγια τριας γνωσις ουσιωδης εστιν ανεξιχνιαστος τε και ακαταληπτος. ου γαρ συμφερει αποβλεπειν εις τους των εθνων σοφους τον θεον περιεκτικως καλουντας πνευμα γνωστικον και πυρ ως δη ουκ εχοντα ειδος αλλα μεταβαλλομενον τε και ομοιουμενον οις αν θεληι. τουτο δε θεου ουχ οικειον αλλα των δαιμονων των υπερβαλλοντως υπ' εχεινων σεβομενων, εν πασι δε τοις σωματικοις ευχερως μεταμορφουμενων Syriac transmission available in the linked TAN/TEI source. The complete corpus is Syriac-transmitted; Greek survives only fragmentarily, so this display text is a retroversion witness.