Pope Innocent III began to fight against heresy within the church in the year 1215 A.D. Heresy was a serious problem that threatened the beliefs and security of the Church. Heresy was defined as denying the basic beliefs and principals of the gospel.
If a heretic could not be converted, then they would be excommunicated, or kicked out of the Church. Since Christians believed that certain ordinances, such as baptism and partaking of the holy sacrament, were necessary in order to receive salvation, being excommunicated from the Church was the same as having your soul damned from heaven.
One group of heretics living in Germany, France and Italy in the 1200s A.D., were the Poor of Lyon, or the Waldensians. The Waldensians were disturbed by the growing wealth of the clergy, and believed that good Christians would turn away from wealth and live a simple life of poverty.
The Church fought with great energy to convert the Waldensians from their heresy, and eventually excommunicated many of them.