<p>Caravaggio was the greatest artist since Titian, a favourite of Popes and wealthy bankers. But at a time when the resurgent Ottoman Empire was planning a second wave of conquest, he discovered a secret so dark that it threatened the very existence of the Catholic Church. The secret endures. Four hundred years later, Declan O'Malley, the first Irish-born Superior General of the Society of Jesus, learns that his friend, the German Cardinal Horst...