Pope Francis officially became the 266th leader of the Roman Catholic Church's 1.2 billion worldwide followers yesterday during his formal installation Mass in St. Peter's Square. Formally known as Cardinal Jorge Mario Bergoglio, the 76-year-old Argentinean is the first non-European pope in more than 1,200 years and the first from South America. During the Mass, which was attended by an estimated 150,000 to 200,000 people, Francis pledged to serve ââthe poorest, the weakest, the least important,ââ striking the same tones of humility that have marked the days since his election. -- Lloyd Young ( 24 photos total