The Councils

For as long as I can remember, I have believed that the Trinity is three distinct persons in one divine essence, rather than three gods or three personalities or functions of God.
But if anyone had asked me to point to the Bible verse or verses that spell out this precise understanding of God, I would have been at a loss. In fact, there is no place in the Bible that actually does specifically define the Trinity in this way. Nor for that matter is there to my knowledge anywhere in the Bible that explicitly lays out the belief that Christ is fully human and fully divine.
Rather these doctrines, and many others, were defined by the Church through a series of seven churchwide, or “ecumenical,” (from οἰκουμένη, meaning "the inhabited world") councils during the first eight centuries. The purpose of the councils was to condemn heresy, not to provide an exhaustive explanation of Christian Doctrine. As Clark Carlson writes in his book The Truth,

All early conciliar definitions of dogma were the response to specific heresies. The Church defined the doctrine of the Trinity in response to Arianism and Eunomianism, and even then the Church never pretended to define the "whole truth" about the mystery of the Trinity. Similarly, the Christological definitions of the later councils were occasioned by the heresies of Nestorianism, Monophysitism, Monothelitism, and Iconoclasm. Nowhere did the Fathers of these councils claim to have exhausted the "whole truth" about the mystery of the Incarnation. These definitions never ever presumed to explain these mysteries, but to exclude false opinions. They were not so much positive statements as negative ones (p.163)

1. First Council of Nicaea:
Held in A.D. 325 in Nicaea, Asia Minor; Convened by the Emperor Constantine the Great; Attended by 318 bishops.

Condemned Arianism and attempted to standardize Easter.

Adopted the original Nicene Creed, devising the term homoousios (from the Greek όμού meaning “same” and ουσία meaning “essence or being”) to describe Christ’s relationship to the Father (contrary to homoiousios: from όμοιος meaning “similar” and ουσία meaning “essence or being.”)

This and all subsequent councils are not recognized by nontrinitarian churches: Arians, Unitarians, Latter-day Saints and members of other Mormon denominations, and Jehovah's Witnesses.

Key figures:
Alexander of Constantinople, bishop
Eusebius of Caesarea, historian
Athanasius of Alexandria, theologian
Arius of Alexandria, priest

2. First Council of Constantinople:
Held in A.D. 381 in Constantinople, Asia Minor; Convened by the Emperor Theodosius the Great; Attended by 150 bishops.

Condemned Macedonianism and Apollinarianism; Defined the doctrine of the Holy Spirit; decreed that there was one God in three persons.

Revised the Nicene Creed into the present form used in the Eastern and Oriental Orthodox churches and prohibited any further alteration of the Creed without the assent of an Ecumenical Council.

Key figures:
Basil of Caesarea, bishop and theologian
Gregory of Nyssa, bishop and theologian
Gregory of Nazianzus, bishop and theologian
Macedonius I of Constantinople, bishop
Apollinaris of Laodicea, bishop

3. First Council of Ephesus:
Held in A.D. 431 in Ephesus, Asia Minor; Convened by the Emperor Theodosius II (grandson of Theodosius the Great); Attended by 200 bishops.

Condemned Nestorianism; proclaimed that Jesus was fully divine from conception, thus naming the Virgin Mary the Theotokos (Greek Θεοτόκος, "God-bearer" or more commonly "Mother of God"); Declared that Jesus Christ is one person, not two separate "people": the Man, Jesus Christ and the Son of God, complete God and complete man, with a rational soul and body.

Affirmed the text of the "Creed" decreed at the First and Second Ecumenical Councils to be complete and forbade any addition or deletion.

This and all following councils are not recognized by the Assyrian Church of the East.

Key figures:
Cyril of Alexandria, bishop
John Chrysostom, bishop
Nestorius of Constantinople, bishop

4. Council of Chalcedon:
Held in A.D. 451 in Chalcedon near Constantinople; Convened by the Emperor Marcian; Attended by 630 bishops.

Condemned the doctrine of monophysitism; described and delineated the "Hypostatic Union" and affirmed the two natures of Christ: human and divine.

Adopted the Chalcedonian Creed.

Recognized Constantinople as the “New Rome” and granted Jerusalem the fifth position of honor within the “Pentarchy”: 1) Rome, 2) Constantinope, 3) Alexandria, 4) Antioch, 5) Jerusalem.

This and all following councils are not recognized by the Oriental Orthodox.

Key figure:
Leo I of Rome, bishop

5. Second Council of Constantinople:
Held in A.D. 553 in Constantinople; Convened by the Emperor Justinian the Great; Attended by 165 bishops.

Reaffirmed decisions and doctrines explicated by previous Councils, condemned new Arian, Nestorian, and Monophysite writings, decreed Theopaschite Formula (“God suffered in the flesh”).

Key figures:
Eutychius of Constantinople, bishop
Vigilius of Rome, bishop

6. Third Council of Constantinople:
Held in A.D. 680 in Constantinople; Convened by the Emperor Constantine IV; Attended by 170 bishops.

Condemned monothelitism, affirmed that Christ had both human and divine wills, that He acted as God only: that His divine will made the decisions but His human will carried them out.

Key figure:
Maximus the Confessor, theologian

7. Second Council of Nicaea:
Held in A.D. 787 in Nicaea; Convened by the Empress Irene; Attended by 367 bishops.

Condemned iconoclasm, upheld the display and veneration of icons.

Addressed the broader issues of the character of Christ's human nature, the Christian attitude toward matter, and the true meaning of Christian redemption and the salvation of the entire material universe.

Key figures:
Tarasios of Constantinople, bishop
John of Damascus, theologian and hymnographer

For a more thorough treatment of these councils, I highly recommend this excellent series of lectures.