Among the key events in the development of Christianity are the Council of Nicaea in 325 A.D., the first Crusade in 1097 to 1099, events of the Protestant Reformation, and the Council of Trent in the 16th century. A short timeline of Christianity that includes both Catholic and Protestant landmarks is available from OurCatholicFaith.org.
The First Council of Nicaea convened in response to the Arian heresy, a heresy that taught that Jesus was not God or equal to God, but was a lesser, though still-exulted, creature. The Council of Nicaea rejected this opinion and reaffirmed the full divinity of Jesus. The council also drew up the Nicene Creed.
The first Crusade took place between 1097 and 1099. Among the original aims of the Crusades were to take back sites in the Holy Land and enable Christian pilgrims to have safe passage to these sites.
Major events of the Protestant Reformation include Martin Luther's purported posting of his 95 theses at Wittenberg in 1517 and King Henry VIII's break from the Roman Catholic Church in 1534. The Catholic Church responded to the Reformation with its own program of reform, epitomized in the Ecumenical Council of Trent (1545 to 1563). The council reaffirmed and further developed church teaching and fought abuses that had crept into the Catholic Church.