What is one reason that the church was so poweful during medieval times?

The Catholic Church put forth the belief that people could only get to heaven through the Church. Most people in the time period donated 10 percent of their income to the Church, giving it incredible wealth, and the Church did not pay taxes. On top of regular tithing, parishioners often had to pay for ceremonies, such as baptisms, and they paid penances also. Others gave land to the Church, meaning it held about 30 percent of Western Europe in land. The other part of the Church's power came from the fact that most parishioners were illiterate, which meant they had to rely on the Church to interpret the biblical texts

Because the church and kings or queens if any were in royal positions, they collected taxes. :)