Its story
Once the Son's divinity gained ground, the question of the Holy Spirit arose. The Nicene Creed (325) said only 'and in the Holy Spirit'. Around 360 in Egypt, Christians whom Athanasius called 'Tropici' held that the Spirit was a superior angel or a creature. In Asia Minor, a group led by Eustathius of Sebaste accepted the Son as like or equal to the Father but refused to call the Spirit God. Their opponents called them Pneumatomachi ('Spirit-fighters').
They were later associated with Macedonius, bishop of Constantinople deposed in 360, hence the name 'Macedonians', though historians doubt that Macedonius played any real doctrinal role in the movement. What is known of them comes mostly from critics.
The answer was one of the great theological achievements of the 4th century. Athanasius, in his Letters to Serapion (c. 359–361), argued that the Spirit sanctifies and deifies, which no creature can do. Basil of Caesarea, in On the Holy Spirit (c. 375), defended the doxology 'glory to the Father with the Son together with the Holy Spirit', drawing on Scripture and liturgical practice, though he avoided explicitly calling the Spirit 'God' so as not to alienate moderates. Gregory of Nazianzus, in his Fifth Theological Oration (380), affirmed it outright.
The Council of Constantinople (381) condemned the Pneumatomachi (canon 1) and expanded the article on the Spirit: 'the Lord, the giver of life, who proceeds from the Father, who with the Father and the Son is worshipped and glorified'. This text, the Niceno-Constantinopolitan Creed, is today shared by Catholics, Orthodox, Oriental Orthodox and most Protestants; the later Latin addition of the Filioque ('and the Son') in turn became a point of division between East and West.