Final answer:
Pope John XXIII is renowned for convening the Second Vatican Council, which modernized many practices of the Catholic Church and altered its approach to ecumenism, liturgy, and engagement with the modern world.
Step-by-step explanation:
The contribution of Pope John XXIII to the development and expression of Christianity was significant, though the information provided seems to focus on other historical figures within the church like Gregory the Great, Pope Martin, and the situation during and after the Great Schism. However, Pope John XXIII's most noted contribution was the convening of the Second Vatican Council (Vatican II), which aimed at modernizing the practices of the Roman Catholic Church. His efforts in initiating this council significantly influenced the direction of the Catholic Church, fostering ecumenical relations and revising liturgical practices. Vatican II led to numerous changes, including mass being conducted in local languages rather than just Latin, a more engaging role for laypeople in church services, and a new approach to relations with other religions and the modern world.