The Holy Spirit And The Trinity

Εἰ δέχεται ὁ πρὸς σὲ διενεχθεὶς αἱρετικὸς, καθὼς γέγραφας, τὸν τοῦ Θεοῦ Υἱὸν δογματιστὴν ἀξιόπιστον, αὐτὸς διδάσκει τὴν οὐσίαν τοῦ προσκυνητοῦ τῆς ἐνότητος Πνεύματος, ἕν ὄνομα τῆς θείας Τριάδος ἀποφηνάμενος, τῆς μιᾶς οὐσίας σημαίνων τὴν ἕνωσιν. Εἰ δὲ αὐτῷ μὴ πιστεύει, τὴν ἀκρίβειαν τοῦ συγγενοῦς συνμαίνοντι Πνεύματος, καὶ τὰ ὑπὲρ νοῦν ἀνθρώπινον ἐκδιδάσκοντι, περιττὴ τυγχάνει τῶν ἡμετέρων λόγων ἡ σύστασις· ἐπειδὴ τὸ γεῶδες σκῆνος μόλις εὑρίσκει τὰ ἐν ποσί. Τὰ δὲ ἐν οὐρανῷ ἀπόκρυφα, καὶ τὴν ἡμετέραν αἴσθησιν ὑπερβαίνοντα, αὐτὸς καὶ ἀποκαλύπτει, καὶ ἐπίσταται ὁ κατοικῶν τὸν οὐρανόν. Ἅγιος Ἰσίδωρος Του Πηλουσιώτου, Βιβλίον Πρῶτον, Ἐπιστολὴ Κ´ Ἱερακι Λαμπροτατῳ Source: Migne PG 78.196a

If that dissentient heretic who is with you, as you write, receives the Son of God's teaching as worthy of belief, it is He who teaches the essence of the Spirit in the adorable union, pronouncing with one name the Holy Trinity, 1 which signifies the the union of the one substance. And if he does not believe in it, that with certitude the cognate nature of the Spirit is signified there, it is beyond the rigour of our words to teach those things which are beyond the human mind, which can scarcely know the things of this terrestrial tabernacle before our feet. Heavenly secrets, which exceed our senses, He well knows how to reveal and teach who dwells in heaven. Saint Isidore of Pelusium, Book 1, Letter 20, To The Illustrious Hierax 1 Mt 28.19