Byzantium by Stephen R. Lawhead

An Irish monk from Kells is chosen to carry a magnificent illuminated gospel to the emperor in Constantinople, but his mission is derailed when Viking raiders capture him and draw him into their world, leading him by long river routes to the fabled city. There he is swept into the splendor and intrigue of the imperial court, confronting clashing cultures, betrayal, and a profound crisis of faith. His journey—from Norse longhouses to Byzantine palaces and the Holy Land—becomes a transformative pilgrimage testing courage, loyalty, and the meaning of belief.