All Christians should be aware of the ancient councils that provided clarification on doctrine according to Sacred Tradition. It is important to know what heresies the Church has dealt with in the past. (Heresies tend to "reincarnate" later in society in various shapes and forms.)
Here's a brief outline of the first seven councils:
(1) The Council of Nicea in 325 A.D.
Heresy: Arianism (Denied the divinity of Christ)
The Council declared that Jesus is true God and true Man.
(2) The Council of Constantinople in 381 A.D.
The Council reaffirmed the previous Council of Nicea and also combated Macedonianism which denied the divinity of the Holy Spirit and believed He was a creation of the Son. The work of these two first councils is where the Church wrote the Nicene Creed. (Officially the Nicea-Constantinople Creed.)
(3) The Council of Ephesus in 431 A.D.
Nestorius believed that Christ was two persons. Mary was not the Mother of Christ as God but Christ as Man. She was therefore not the "Theotokos" (God-bearer in Greek, Mother of God) but the "Christokos:" only the Mother of Christ as Man. The Council affirmed that Christ is ONE DIVINE PERSON with no division. Thus, Mary is the Mother of God.*
*As the Word made flesh, not the Mother of God the Father, Creator of the Universe. (I lament the fact that this asterisk is even necessary for Christians.)
(4) The Council of Chalcedon in 451 A.D.
Monophysitism taught that Christ has one nature, a divine nature, that swallowed up the human nature. The Council affirmed that Christ has two natures: one human and one divine. This is also known as the Hypostatic Union in Christ. He is one person with two united natures: human and divine.
(5) The 2nd Council of Constantinople in 553 A.D.
Heresy: Three Chapters (Nestorianism)
The "Three Chapters" were theological writings which were tainted by the Nestorian heresy. The Council reaffirmed the teachings of the Council of Ephesus and Chalcedon.
(6) The 3rd Council of Constantinople in 680 A.D.
Monothelitism taught that Christ only had one will; that the human will was absorbed by the divine will. The Council affirmed that Christ has two wills, human and divine, in accordance with His two natures.
(7) The 2nd Council of Nicea in 787 A.D.
Heresy: Iconoclasm, Adoptionism
Iconoclasm taught that sacred images were idolatrous. Adoptionism taught that Christ was not the Son of God by nature, but only through adoption. The Council affirmed that images in sacred art are not idolatrous. It also reaffirmed the hypostatic union in Christ.
The martyrs DIED to preserve orthodoxy in theology. The least we can do is KNOW it. The current notion that theology doesn't matter must be abandoned.