The Nicene Council only concluded that the Father and Son are ontologically one: it did not include the Holy Spirit in the co-substantial relationship supposedly obtaining between the Father and Son. There was simply an implication that the Holy Spirit was in some way associated with the Godhead.
Yes there was an affirmation of belief in the Holy Spirit, but the Nicene Creed did not put forth a triune statement about God. It would take another fifty-six years and more “heretical” developments, before the relationship between the Father, Son and Holy Spirit was clarified.
Although activities were predicated of the Holy Spirit that can only be predicated of God, the Trinity was still not explicitly called God. Further addenda or adjustments would be made before the church would explicitly state that the holy spirit was equal to the Father and the Son.
One early witness who testifies to these early developments regarding the Trinity doctrine is Gregory of Nazianzus. In the work _Epistles 58_ Gregory Nazianzus explained the absence of the Holy Spirit from the ancient discussions about the Godhead, by stating that “the Old Testament proclaimed the Father manifestly, and the Son more hiddenly. The New [Testament] manifested the Son and suggested the deity of the Spirit. Now the Spirit himself is resident among us, and provides a clearer explanation of himself.” As late as 380, he wrote, “to be slightly in error [about the Holy Spirit] was to be orthodox.” This statement too proves that the orthodox understanding of the Holy Spirit was not “clear” until 381. As a matter of fact, this statement further demonstrates that the church neither subscribed to nor affirmed the teaching of the Trinity until 381 C.E. It is clear that the “details” of the Trinity still had to be worked out (The Christian Tradition, Jaroslav Pelikan, Vol. I, p.
213. Cf. also Gregory Nazianzus–Orations 31.5).
From a brief look at these developments, it seems warranted to conclude that the NT does not present a clear expression of the Triune Godhead. Therefore, we could reasonably conclude that neither the primitive church nor the ante-Nicene fathers taught the Trinity. Gregory Nazianzus even proclaimed that Scripture did not, “very clearly or very often call him [the Holy Spirit] God in so many words, as it does the Father and later on the Son” (Gregory Nazianzus, Orations 31.12).
Gregory’s testimony is so important because he lived at the time when the Trinity assimilated its way into Christian didache. Concerning this prominent Christian “father,” Jaroslav Pelikan says: “In remarkable summary of the controversy within the orthodox camp, composed in the same year, he [Gregory Nazianzus] declared: “Of the wise men among ourselves, some have conceived of him
[the Holy spirit] as an activity, some as a creature, some as God; and some have been uncertain which to call him . . . And therefore they neither worship him nor treat him with dishonor, but take up a neutral position.” He did add, however, that “of those who consider him to be God, some are orthodox in mind only, while others venture to be so with the lips also.”