Martyr's Fire by Sigmund Brouwer

Will this dangerous quest lead the outcast Orphan King toward an ancient secret—or to certain destruction? Posing as a beggar, Thomas escapes Magnus after fifteen men, who are calling themselves the Priests of the Holy Grail, arrive and take control of the castle through wondrous acts and apparent miracles. With the help of his longtime friend Gervaise, Thomas sets out on a journey that leads him to the ancient Holy Land. Unaware that Katherine and Hawkwood are watching over him, Thomas is tested in his beliefs and comes face to face with the ancient power that the Merlins and Druids have long been searching for. Enter the world of Merlin’s Immortals, where ancient secrets and evil conspiracies take you on a breathless adventure of discovery, intrigue, and hidden knowledge.