(Gregory of Nazianzus (ca 329 - 390 AD) was Archbishop of Constantinople who presided over Ecumenical Council of Constantinople in 381 AD. He was one of the main architects involved in expansion of the clause concerning Holy Spirit in the Niceno-Constantinopolitan Creed. He was one of the major defenders of Trinitarian theology through his famous theological orations and is rightly called Gregory the Theologian. He is considered one of the Cappadocian fathers (along with Basil of Caesarea and Gregory of Nyssa). This is taken from his Theological Oration 25 given in honor of Maximus I of Constantinople, where he clearly explicates Trinitarian faith)
Define our faith by teaching the knowledge of One God, Unbegotten, the Father; and One Begotten Lord, His Son; and One Holy Spirit, who proceeds or goes forth from the Father, to those who understand things properly—combated by the impious but understood by those who are above them, and even professed by those who are more spiritual;
Teach that we must not make the Father subject to another source, lest we posit a ‘‘first of the First,’’ and thus overturn the divine existence; nor should we say that the Son or the Holy Spirit is without source, lest we take away the Father’s special characteristic. For they are not without source—and yet in a sense they are without source, which is a paradox. They are not without source with respect to their cause, for they are from God even if they are not subsequent to him in time, just as light comes from the sun. But they are without source with respect to time, since they are not subject to time. And teach that we do not believe in three first principles, lest we espouse the polytheism of the Greeks; nor in a solitary principle, Jewish in its narrowness and somewhat grudging and ineffectual, either by positing a self-absorbing Deity or by casting down the natures of the Son and Spirit and making them foreign to Divinity—as though Divinity feared some rival opposition, or was able to produce nothing higher than creatures!
Teach that the Son is not unbegotten, for the Father is unique; and that the Spirit is not Son, for the Only-Begotten is unique, the result being that they each possess this divine quality of uniqueness, the one sonship and the other procession, which is different from sonship. Rather, teach that the Father is truly a father—much more truly even than human fathers are—because He is a Father uniquely and distinctively, in a way different from corporeal beings; unique, being without a mate; of one who is unique, namely the Only-Begotten; He is only a Father, since He was not formerly a Son; completely a Father and father of One who is complete; and Father from the beginning, since He did not become a Father at a later point in time. Teach that the Son is truly a son, because he is Son alone, of One alone, absolutely, and only, since He is completely a Son, and of One who is complete, and from the beginning, having never come to be a Son, since His Divinity is not due to a change of purpose, nor His divinization to moral progress, otherwise there would be a time when the One was not a Father and the Other was not a Son.
Teach that the Holy Spirit is truly holy, because there is nothing else that is like it or holy in the same way. Its sanctification does not come by way of addition, but it is holiness itself, It is neither more or less; it did not begin, nor will it end, in time. In effect, common to Father, Son, and Holy Spirit is the fact that they were not created, as well as their Divinity. Common to the Son and Holy Spirit is the fact that they come from the Father. Uniquely characteristic of the Father is unbegottenness; of the Son begottenness; and of the Spirit being sent.
