The Schism was a split between religious groups in Lustria over 700 years ago. There had been a slow build up of dissent leading up to the Schism that scholars now point to as an obvious indication of the rebellion, and yet contemporary figures seemed to be keen to ignore these warning signs- perhaps it's simply true what they say, everything is clearer in hindsight.
Some scholars pin the start of the rebellion as early as Jacques du Miles' speeches on the nature of the gods and the path to heaven, but given the 60 or so quiet years between that and the persecutions, most scholars do not. Du Miles' speeches rallied a portion of the Lustrian population who had become disenfranchised by the religious status quo. They saw stagnation and corruption in the church and felt like the way they worshipped wasn't the way the gods intended. Du Miles', a famous theologian in Lustria, convinced them that the gods grant salvation to everyone, and that the church organization is therefore useless.
The persecutions of the Mortalists by the Fundamentalists began, as mentioned, about 60 years after Jacques du Miles' speeches. A decent group had split from the Fundamentalist church. These so called Mortalists believed that they were guaranteed access to heaven simply by being creatures of the gods. The Fundamentalists saw this as a direct threat to the church and began fining, exiling, or killing Mortalists depending on the severity of their heresy.
Many people on both sides preached for peace, but their voices were lost in the din. Rebellion was coming whether they wanted it or not. As the pogroms grew worse, Mortalists gathered in the streets of Amia, demanding an end to their persecution, the army was called in, and, as expected, many Mortalists were killed by the state. After a series of bloody uprisings, the Mortalists looked like they were going to be destroyed - the Lustrian army was making short work of their 'soldiers', who were mostly untrained civilians with farming instruments. But, destruction wasn't their fate. Their leader organized a mass exodus of Mortalists, taking them across Morgrave to the great forests and marshes north of Qemt'Or, claimed only by the reclusive Elves and Dwarves...