What is the mane of the movement to change the christian church for the better that ended up dividing Christianity?

Oddly, the answer is "The Reformation" or "The Protestant Reformation" which officially started in 1517 when Martin Luther nailed his "95 Theses" to the door of the Castle Church in Wittenberg, Germany. The split had a great deal to do with the Renaissance period in which it was taking place, as much conventional wisdom was being questioned during that time, but the driving motivation behind the Reformation was over theological differences. Some of it was based on fervent and growing nationalism, and over corruption in the Curia, the ecclesiastical court which assisted with the governance of the Roman Catholic church. The practice of selling "indulgences" was a major influencer in the Reformation.