In this photo gallery of three of the holiest places in the world, visit the Sultan Ahmed Mosque (Blue Mosque) in Istanbul, the Grand Mosque of Mecca, and the Dome of the Rock in Jerusalem.
Christianity is by far the largest world religion with 2.18 billion followers. Of that number, more than half are Catholic. Learn more about the teachings of Christianity in Istanbul, where Charlie met with Patriarch of the Armenian Orthodox Church to discuss faith and hope. And visit Rome for a rare chance to attend a service at