Baptism And The Trinity

Ἔοικάς μοι βούλεσθαι τῷ Κυρίῳ πείθεσθαι, καὶ τὰς Γραφὰς ἐρευνᾷν. Ἀνάγκη τοίνυν καὶ ἀποκρίνεσθαι πρὸς τὰς πεύσεις σου. Οὕτος ἐστιν ὁ Υἱός μου ὁ ἀγαπητὸς, ὁ Θεὸς καὶ Πατὴρ ἐξ οὐρανῶν βαπτιζομνένῳ τῷ Υἱῷ ἐπεβόησεν, ἵνα καὶ τὸν γνήσιον δείξῃ καὶ φυσικὸν Υἱὸν, μεταξὺ τῶν θέσει καὶ ἀξίᾳ χρητματιζόντων υἱῶν ἀμφιβαλλόμενον, καὶ τὴν θείαν καὶ προσκυνητὴν τῆς θεότητος Τριάδα φανέρώσῃ, ἐν ἰδίαις ὑποστάσεσι γνωριζομένην· καὶ Μοντανῷ καὶ Σαβελλίῳ ἀποκλείσῃ τὴν κακόνουν ἀσέβειαν. Τοῡ γὰρ Υἱοῦ βαπτιζομένου ὁ Πατὴρ ἐμαρτύρησε, καὶ τὸ Πνεῦμα τὸ ἅγιον καταπτὰν ὁμοούσιον ἑαυτῷ καὶ τῷ Πατρὶ ὑπεδείκνυ τὸν βαπτιζόμενον Ἅγιος Ἰσίδωρος Του Πηλουσιώτου, Βιβλιον Πρῶτον, Ἐπιστολή ΞΖ’ Τιμοθεῳ Ἀναγηωστῃ Source: Migne PG 78.228a

It seems to me you are scrutinising the Scriptures in order to be persuaded about something regarding the Lord. Thus concerning this about which you enquire it is necessary for me to reply. 'This is my beloved Son,' 1 says God the Father from heaven with a loud voice, giving witness to His Son and showing that He is the true and natural Son among those who are called sons through adoption and dignity, 2 and this also declares the holy and worshipful Trinity of Divinity, with each one being known as their own person, which is denied to the depraved impiety of Montanus and Sabellius. For with the Father bearing witness to the baptised Son, and the Holy Spirit coming down, this reveals that the one baptised is of the same substance as the Father. Saint Isidore of Pelusium, Book 1, Letter 67, to Timotheus the Lector 1 Mt 3.17 2 Jn 1.12, Rom 8.15, Galat 4.7, Ephes 1.5