When the Roman emperor Constantine I converted to the Christian faith (out of gratitude for military victory through what he believed to be divine intervention), the persecution of the religion was put to an end at the Edict of Milan (AD 313) and later made to be the state religion by emperor Theodosius I (AD 380). Therefore, from the time of Constantine (and Theodosius) onward, the interests of the Christian Church were the interests of the empire. This sentiment led to the necessity of promoting the true and orthodox faith over-and-against various heretical manifestations and troublemakers, while also establishing and maintaining various rules or “canons” to govern and organize the now-globalized faith.
The first Council of Nicaea (AD 325) was convened by Constantine I (shortly after purposing to move the capital of the empire from old Rome to Constantinople/Byzantium) to primarily deal with the heresy of Arius (a priest in Alexandria who taught that the Son was inferior to the Father and created) and the organization of the Church (establishing the priority of the sees of Rome, Alexandria and Antioch, with Jerusalem still in subjection to the Metropolitan of Caesarea and Constantinople not yet officially inaugurated as the capital city). The Council maintained that Christ was “one with the Father” (homoousios) and composed a Creed to preserve the confession of orthodox Christianity.
The Council of Constantinople (AD 381) expanded the Nicene Creed by adding a clause on the role of the Holy Spirit as a Person of the holy Trinity (same in essence and fully God along with the Father and the Son), who “proceeds from the Father” and “with the Father and the Son together is worshipped and glorified.” The Council also made the bishop of Constantinople to be second in honor to that of old Rome (pushing Alexandria down to third in honor), making Constantinople “New Rome” (Canon III).
After 381, Arianism died out for the most part, but the third Canon of this council had long-lasting repercussions, leading to nearly 70 years of controversy between both Constantinople and Alexandria. At a synod convened by Theophilus of Alexandria in AD 403, John Chrysostom (“Golden Mouthed”) was exiled for the first time from Constantinople (falsely accused of Origenism, with Theophilus being a staunch opponent of Origen’s teachings). Later, at the Third Ecumenical Council in Ephesus (AD 431), the successor of Theophilus in Alexandria (Cyril) had Nestorius of Constantinople condemned as a heretic for his insistence on separating the humanity of Christ from his divinity, while also over-emphasizing Christ’s humanity. Alexandria had won two “victories” over Constantinople, as a result. Unfortunately, the Alexandrians went too far with their teaching about Christ’s nature and held a second council in Ephesus (AD 449), led by Dioscorus (successor to Cyril), where they taught that Christ had only one nature (physis) after the Incarnation. This teaching ignores the “balancing” concessions Cyril had made on the issue in AD 433, as a means by which to preserve unanimity with the Antiochenes.
This “too far” position became known as the “Monophysite” heresy, and led to the Fourth Ecumenical Council in the city of Chalcedon (AD 451), being convened by the emperor Marcian. This council supported more of an Antiochene position on the nature(s) of Christ, especially as articulated by Pope Leo I of Rome in his Tome. The see of Jerusalem was also “freed” from the jurisdiction of Caesarea by this council, instituting the “Pentarchy” of Orthodoxy (the five sees of the Church, each led by a Patriarch). Incidentally, the Church of Cyprus was given autonomy and independence at this council, as well.
The “Definition of Chalcedon” was later clarified and more deeply understood through the work of the Fifth Ecumenical Council (AD 553) in Constantinople, using a more Alexandrian viewpoint to explain how the two natures of Christ unite as a single Person. This council was also called in order to condemn the heretical (Nestorian) opinions of three deceased bishops (Theodore of Mopsuetia, Theodoret of Cyrrhus, and Ibas of Edessa). The Monophysites, who were condemned by Chalcedon, objected that these three bishops were not condemned by Chalcedon as Nestorians. This council, at Justinian’s request, was to ensure no oversight remained on this issue (leaving the Monophysites no room to accuse the orthodox of advocating Nestorianism through these bishops and their beliefs). This council also condemned the teachings of the third century presbyter Origen.
The Sixth Ecumenical Council, also held in Constantinople (AD 680-681), was convened by emperor Constantine the New primarily to condemn the Monothelite heresy. This heresy taught that — despite having two natures (as one person) — Christ only had one will. This diminished the humanity of Christ by teaching that he was a man without a will (and therefore, unlike us and incapable of redeeming humanity by the incarnation, death and resurrection of himself, with him not being “truly” human, just as we are). In addition to the condemnation of Monothelitism, the council anathematized as heretics Pope Honorius I of Rome, Sergius I of Constantinople, Cyrus of Alexandria, Paul II and Peter of Constantinople, and Theodore of Pharan for their part in propagating the heresy of Monothelitism.
From the seventh century onward, the spread of the new religion of Islam by the sword let to the captivity of a large portion of the Christian east, including the Patriarchates of Jerusalem, Antioch and Alexandria, with Constantinople itself being close to capture. These incursions spread to the west and into modern-day Europe, as well. One of the characteristics of Islam was its anti-Incarnational rejection of images (Icons), and three years prior to the first wave of iconoclasm (circa AD 726-787, a heresy that rejects the usage or veneration of Icons, relics, etc.), the Muslim Caliph Yezid decreed that all Icons in Muslim-controlled lands should be eliminated.
The Roman emperor Leo III promoted iconoclasm in the empire (no doubt fearing further Muslim incursions) starting in AD 726, and this heresy prevailed until the empress Irene convened the Second Council of Nicaea (Seventh Ecumenical) in AD 787. This council upheld the iconodule position, allowing the continued veneration of both Icons and relics (as well as their usage in the adornment of Christian buildings, private homes and monasteries). While iconoclasm would again be promoted by emperor Leo V the Armenian (AD 815), in a probable attempt to curry favor from the Franks (and their armed forces) in the face of both Islamic and Bulgarian conquest, it would ultimately be defeated by the empress Theodora at a synod in Constantinople in the year 843 (celebrated as the “Triumph of Orthodoxy”).