The Arian era of church history is probably the most fascinating to me. So much was at stake in how people would view both Christ and the Holy Spirit, and we see the Church unite more than ever.
Unitarianism takes two extremes: modalism and Arianism. Modalists (I sympathize with them, but know they're wrong) are so protective of the doctrine of Christ's deity that they forget to distinguish him from God the Father and the Spirit.
In 318, Alexander, bishop of Alexandria, preached about "the Great Mystery of the Trinity in Unity." Arian listened to it in horror. He thought it made God sound like a polytheism. He then developed the most polytheistic theology that tries to pass for Christian. He believed that Christ was divine, but a lesser being than the Father.
His views became so popular that they divided the Roman empire, and Constantine called for an ecumenical council to solve the rift.
My hero, Athanasius, rose up to show how Christ could not have completely saved us had he not been completely equal to God the Father. His boldness caused his five exiles.
Eusebius of Caesarea, bless his heart, tried a compromise. Christ was of like (homoi) or similar essence to the Father. Athanasius protested saying Christ is the same (homoousios) essence with the Father.
So Constantine called ordered the Council of Nicaea in 325. There, Athanasius fought tirelessly until the Church universal declared that Christ is completely God and completely man. Now that that was settled, Constantine and his sons went along with Arian theology just the same. So the Church met again in Constantinople in 381 to say, no, really, Christ is God and man. The Nicaean faith will not be "set aside but shall remain dominant."
As for the Holy Spirit, Macedonius tried to teach that He was more like the angels, a minister and servant. If this would true, then we truly would have been left as orphans when Christ ascended to heaven. But it is necessary that the Spirit is equal to the Father and the Son, even if he does proceed from them. We could not stay in the faith without Him.