Short answers, pulled from the story.
Christianity began in the first century within the Roman province of Judaea as a sect of Second Temple Judaism. An early Jewish Christian community formed in Jerusalem under the leadership of James the Just, Peter, and John.
Paul the Apostle solved the tension between Jewish law and Gentile inclusion by preaching that salvation came through faith in Christ and baptism. Before his conversion, Paul had persecuted these early believers but became a missionary to the gentiles after a dramatic experience.
Emperor Constantine issued the Edict of Milan in 313 AD which decriminalized Christianity throughout the Roman Empire. This support grew steadily until his own baptism on his deathbed and the convening of the First Council of Nicaea in 325 AD.
The most famous division occurred in 1054 when the Eastern Orthodox Church separated from the Catholic Church in what historians call the East-West Schism. Disagreements centered on administrative liturgical and doctrinal issues especially papal supremacy which Eastern Orthodox leaders opposed.
Martin Luther initiated a major transformation of Western Christianity by posting his Ninety-five Theses in 1517 opposing the sale of indulgences. Printed copies spread rapidly throughout Europe challenging established church authority and leading to the Edict of Worms in 1521 that condemned him.