The church was the largest landowner in Europe at one point, and they also made a lot of money from tithes such as "peter's pence." The church had a lot of material wealth that they were not going to give up due to pure human greed, and the desire to establish the so called "Kingdom of God on Earth" where the Pope and the church were going to reign supreme and bring peace/piety to all the land. The church obviously had to clash with secular rulers in order to obtain this goal, and this led to the investiture conflict [should bishops be appointed by and answerable to the pope or the emperor?] and the truce of god [private warfare was banned on some days by the church].