The Great Catechism, chapters 1-4.
“…the Christian who combats polytheism has need of care lest in contending against Hellenism he should fall unconsciously into Judaism…Thus the mystery of the faith avoids equally the absurdity of Jewish monotheism [the Shema, Deut. 6.4; cp. Mar 12.29], and that of heathen polytheism…From the Jewish doctrine, then, the unity of the Divine nature has been retained: from Hellenism the distinction into hypostases. The Jew convicted from Scripture.”
Oratio theol.,5,26 (= Oratio 31,26):PG 36,161-163.
“The Old Testament proclaimed the Father clearly, but the Son more obscurely. The New Testament revealed the Son and gave us a glimpse of the divinity of the Spirit. Now the Spirit dwells among us and grants us a clearer vision of himself.
It was not wise, when the divinity of the Father had not yet been confessed, to proclaim the Son openly and, when the divinity of the Son was not yet admitted, to add the Holy Spirit as an extra burden, to speak somewhat daringly.
By advancing and progressing “from glory to glory,” the light of the Trinity will shine in ever more brilliant rays.