The first American pope in the history of the Catholic Church, Pope Leo XIV Begins Papacy with Inaugural Mass Calling for Unity, Peace, and Justice. Pope Leo XIV, officially began his papacy on Sunday with a deeply symbolic and moving inauguration Mass in St. Peter’s Square. The 69-year-old Chicago-born pontiff, formerly Cardinal Robert Prevost, delivered a homily that resonated with over 200,000 gathered pilgrims, clergy, and world leaders, calling for a “united church” and a global effort for reconciliation and justice.