Spirit And 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, ἑν ὄνομα τῆς θείας Τριάδος ἀποφηνάμενος, τῆς μιᾶς οὐσίας σημαίνων τὴν ἔνωσιν. Εἰ δὲ αὐτῷ μὴ πιστεύει, τὴν ἀκρίβειαν τοῦ συγγενοῦς σημαίνοντι 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, Ἐπιστολή Κ' Ἱερακι Λαμπροτατῳ Source:Migne PG 78. 195

If the heretic with whom you have contention, as you write you do, receives the Son of God with the teaching worthy of the faith, then that same teaching teaches about the essence of the worshipful unifying Spirit, in the pronouncement of the one name of the Divine Trinity signifying the conjunction and union of the one essence. But if he has no faith in Him, since the sublimities of the united Spirit are things which are beyond the human mind to teach thoroughly, superfluous is the counsel of our words. One finds that the things of the terrestrial tabernacle are scarcely able to be known, and so the things hidden in heaven, which far exceed our thoughts, He reveals who knows, having heaven for His dwelling place. 1 Saint Isidore of Pelusium, Book 1, Letter 20, To Hierax  1 2 Cor 5.4