Aquinas Institute RCIA

The nature of the Trinity and of Christ is dealt with in the first Seven Ecumenical Councils of the Christian Church (before the formal split between East and West in 1054).

(1) Nicea I 325 - The Son is homoousios (same substance), not homoiousios (like substance)with the Father - contra Arius (first draft of the Nicene Creed)
(2) Constantinople I 381 - Condemned Apollinarianism (Christ had a human body and a divine mind), affirmed the deity of the Holy Spirit (final draft of Nicene Creed)
(3) Ephesus 431 - Condemned “Nest”orius, who said separated the divine and human natures of Christ. Affirmed Mary as “Theo-tokos” (Mother of God) not simply "Christo-tokos" (Mother of Christ, the human nature only)
(4) Chalcedon 451 - Christ’s two natures are unmixed, unchanged, undivided, inseparable. Christ is 100% God, 100% Man, not 50/50 (Nestorius) or 100/0 (Gnosticism, Docetism) or 0/100 (Paganism) 99/100 (Arianism). It is intentionally incomprehensible, because if it could fit into our mind it couldn't be God.
(5) Constantinople II 553 - Condemned the “Three Chapters” to gain Monophysite support.
(6) Constantinople III 680 - Rejected Monotheletism (Christ had only one will) and affirmed diotheletism (Christ had two wills, the divine in submission to the human). This is important because if Christ is not completely human, then we are not completely saved.
(7) Nicae II 787 - Declared the veneration (not the adoration) of icons legitimate and an implication of the Incarnation. In other words, if Christ became fully human, then can be painted like any other human can - and we can venerate (but not worship) those paintings.

The later councils are recognized in the Catholic tradition only

(8) Constantinople IV 869 Emperor Basilius I
(9) Lateran I 1123 Pope Callistus II
(10) Lateran II 1139 Pope Innocent II
(11) Lateran III 1179 Pope Alexander III
(12) Lateran IV 1215 Pope Innocent III
(13) Lyons I 1245 Pope Innocent IV
(14) Lyons II 1274 Pope Gregory X
(15) Vienne 1311 Pope Clement V
(16) Constance 1414 Pope Gregory XI1
(17) Basel,Ferrara and Florence 1431 Pope Martin V Pope Eugenius IV
(18) Lateran V 1512 Pope Julius II
(19) Trent 1545 Pope Paul III
(20) Vatican I 1869 Pope Pius IX
(21) Vatican II 1962 Pope John XXIII

More info. on the Catholic Councils.