Following the life of Jesus Christ, some of the most important events in the history of Christianity are the Edict of Milan, the Council of Nicaea, the Protestant Reformation, the Council of Trent and the Second Vatican Council. Each event had a significant influence across boundaries of different Christian traditions.
After enduring years of persecution under the early Roman emperors, the persecuted Church found relief and a certain level of freedom under Emperor Constantine's Edict of Milan in 313. This allowed the Church to become an established part of society, rather than a secretive body of fugitives.
Twelve years later, Constantine and legates of the pope oversaw a gathering of bishops for the Church's first ecumenical council at Nicaea in 325. At stake was the identity of Jesus Christ, as the Arian heresy had declared Jesus to be a mere creature. The Council of Nicaea rejected this opinion and taught that Jesus was God and equal to the Father.
The Protestant Reformation in the 16th century consisted of a number of break-offs from the Roman Catholic Church over issues of doctrine and discipline, and many of these groups persisted. Significant breaks came from Martin Luther in Germany and King Henry VIII in England. The Catholic Church responded with the Council of Trent (1545-1563), which reiterated doctrine but also corrected some abuses in practice.
The Second Vatican Council (1962-1965) was a council of the Catholic Church, but it influenced many Christian traditions, as it gave impetus to Christian ecumenical cooperation and dialogue.