Early onto the Western Schism, there were many early critics, such as John Wytecliff, who would claim that the Bible is the ultimate sense of governance within the Church, translating it into English against Church tradition. This will get him and his followers killed.
There was also Jan Hus, a bohemian priest who wanted to return the Church back to the people, and offer it in English not latin, he too was martyred.
There was Catherine of Siena. A devoted woman to religion, and a mystic. She was the striking point, as her teachings helped people prepare for the Reformation.
Erasmus is a reformist within the Church, and his attacks on Catholicism helped pave way for people to desert. Should also mention the fact the attacks lead the way for one Martin Luther.