Mystici Corporis Christi (June 29, 1943) is a papal encyclical issued by Pope Pius XII during World War II, on the Church as the Mystical Body of Christ. It is one of the more important encyclicals of Pope Pius XII, because of its topic, the Church, and because its Church concept was fully included in Lumen Gentium but also strongly debated during and after Vatican II. The Church is called body, because it is a living entity; it is called the body of Christ, because Christ is its Head and Founder; it is called mystical body, because it is neither a purely physical nor a purely spiritual unity, but supernatural.