Pope Callixtus II, born Guido di Castello, served as the head of the Roman Catholic Church from 1119 to 1124. He is best known for his efforts to resolve conflicts between the papacy and the Holy Roman Empire, particularly through the Concordat of Worms in 1122, which aimed to settle the investiture controversy regarding the appointment of bishops. During his papacy, Callixtus II also supported the Crusades, encouraging Christian military expeditions to the Holy Land. He worked to strengthen the authority of the papacy and promote church reforms, contributing to the development of the medieval church's political and spiritual influence.