In 381 C.E., the newly appointed emperor, named Theodosius, called to order the First Council of Constantinople. This council was smaller in size, compared to its predecessor at Nicea, with only 150 eastern bishops in attendance. The council primarily met in order to readdress lingering Arian views, to debate the doctrine of the Holy Spirit and the doctrine of the Trinity and to confront the views of Apollinarius.
First, the council reaffirmed the results of the Council of Nicea regarding the deity of the Son, thus emphatically closing the door on Arianism and semi-Arianism.
Second, the council determined that what was said about the deity of the Son should be applied to the Holy Spirit. The creedal statement regarding the Holy Spirit states that the Holy Spirit is “the Lord, the giver of life, who proceeds from the Father” and is to be “worshipped and glorified.” Basil the Great and Gregory of Nazianzus were dissatisfied with such statements regarding the Holy Spirit because they thought that the statements were not strong enough in identifying the deity of the Holy Spirit. They preferred an explicit statement that the Holy Spirit was ὁμοούσιος (homoousios) “of the same substance” with God-the Father and God-the Son. Nevertheless, the council agreed on the deity of the Holy Spirit.
Third, the council addressed the Trinity controversy which at one point involved the views of Sabellius who argued that “God as a single monad is manifest in three distinct and successive operations of self-revealing.” Thus, God-the Father revealed Himself in the temporary modes of the Son and of the Holy Spirit. The council decided on the Trinity by proclaiming that the Father, the Son and the Holy Spirit are of one essence (Gr. ούσια, ousia) in three persons (Gr. ὑπόστασες, hypostases defined with Latin personae).
Fourth, the council rejected and condemned the views of Apollinarius who denied that Jesus had a human mind or spirit. He believed that if Jesus was totally divine, his human nature must have been replaced with the divine logos. Thus, Apollinarius repudiated Jesus’ fully human existence. However, the council agreed that the scriptures teach that Jesus was the God-Man—fully God and fully man.
The Council of Constantinople produced the revised Nicene-Constantinopolitan creed that is presently used throughout Christendom and is simply referred to as the “Nicene Creed.”
 T.A. Noble, “Constantinople, Council of” in New Dictionary of Theology: Historical and Systematic (NDT) eds. Martin Davie et al. (Downers Grove, Ill: IVP Academic,2016), EPUB edition.  Noble, “Constantinople, Council of” in NDT, EPUB edition.  H.D. McDonald, “Monarchianism,” in NDT, EPUB edition.  McDonald, “Monarchianism,” in NDT, EPUB edition.  González, The Story of Christianity, Rev. and Updated, 2nd ed. (New York: HarperOne, 2010), EPUB edition, pt. 2, ch. 20, “Gregory of Nazianzus.”  H.D. McDonald, “Apollinarianism,” in NDT, EPUB edition.