History of Christianity
During this era, there were several Ecumenical Councils.  These were mostly concerned with Christological disputes.  The First Council of Nicaea condemned Arianism and produced the Nicene Creed to define the faith. The Council of Ephesus condemned Nestorianism and affirmed the Blessed Virgin Mary to be Theotokos ("God-bearer" or "Mother of God").  Perhaps the most important was the Council of Chalcedon that affirmed that Christ had two natures, fully God and fully man, at the same time.  That meant that Monophysitism was condemned.