The Council of Nicaea had not fully defined the nature of the Ruach HaQodesh. The Council of Constantinople completed the formula: the Spirit was declared co-equal and co-eternal with the Father and the Son, fully completing the three-person, one-substance doctrine. This was again a council decree — not a revelation, not an apostolic teaching. The Nicene-Constantinopolitan Creed became the doctrinal standard of both Eastern and Western Christianity, cementing a Greek philosophical framework as if it were Hebrew revelation.
Council of Constantinople — The Spirit Becomes the Third Person
The Holy Spirit is formally elevated to co-equal status, completing the triune formula by council decree.